Issue Description:

When I compile my project:

https://github.com/rfdickerson/PureURLSession

I have tried both the Jan 24 and Jan 27 toolchains for Ubuntu 16.10:

I get the following crash:

```
rfdickerson@prometheus:~/projects/PureURLSession$ swift build
Compile Swift Module 'PureURLSession' (12 sources)
0 swift 0x00000000038a8948
1 swift 0x00000000038a9086
2 libpthread.so.0 0x00007f5bf427e630
3 libc.so.6 0x00007f5bf2df97ef gsignal + 159
4 libc.so.6 0x00007f5bf2dfb3ea abort + 362
5 swift 0x000000000107e654
6 swift 0x000000000106f3d4
7 swift 0x0000000001076640
8 swift 0x00000000010767ad
9 swift 0x00000000010767ad
10 swift 0x0000000001073ba7
11 swift 0x000000000107c6ec
12 swift 0x00000000010805e4
13 swift 0x00000000013db899
14 swift 0x00000000013c90d0
15 swift 0x0000000001415a68
16 swift 0x000000000107d9e9
17 swift 0x000000000106f3d4
18 swift 0x0000000001076640
19 swift 0x0000000001071779
20 swift 0x0000000001074fcc
21 swift 0x000000000107511e
22 swift 0x0000000001071ddc
23 swift 0x000000000107c6ec
24 swift 0x00000000010805e4
25 swift 0x00000000013db899
26 swift 0x00000000013c8970
27 swift 0x00000000013b0dc6
28 swift 0x00000000013ab88b
29 swift 0x00000000013ab9d7
30 swift 0x00000000013ab794
31 swift 0x0000000000dd1b2c
32 swift 0x0000000000dc5434
33 swift 0x0000000000c31d07
34 swift 0x0000000000c3ce98
35 swift 0x00000000004a7b31
36 swift 0x0000000000464077
37 libc.so.6 0x00007f5bf2de43f1 __libc_start_main + 241
38 swift 0x000000000046171a
33 swift 0x0000000000c31d07
34 swift 0x0000000000c3ce98
35 swift 0x00000000004a7b31
36 swift 0x0000000000464077
37 libc.so.6 0x00007f5bf2de43f1 __libc_start_main + 241
38 swift 0x000000000046171a
Stack dump:
0. Program arguments: /home/rfdickerson/swift-toolchains/swift-DEVELOPMENT-SNAPSHOT-2017-01-24-a-ubuntu16.10/usr/bin/swift -frontend -emit-module /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/TransferState~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/TaskRegistry~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/NSURLSessionTask~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/HTTPMessage~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/NSURLSession~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/NSURLSessionConfiguration~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/EasyHandle~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/MultiHandle~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/NSURLSessionDelegate~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/Configuration~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/libcurlHelpers~partial.swiftmodule /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.build/HTTPBodySource~partial.swiftmodule -parse-as-library -target x86_64-unknown-linux-gnu -disable-objc-interop -I /home/rfdickerson/projects/PureURLSession/.build/debug -enable-testing -g -module-cache-path /home/rfdickerson/projects/PureURLSession/.build/debug/ModuleCache -D SWIFT_PACKAGE -emit-module-doc-path /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.swiftdoc -module-name PureURLSession -o /home/rfdickerson/projects/PureURLSession/.build/debug/PureURLSession.swiftmodule

1. While loading members for '_TransferState' in module 'PureURLSession'
2. While deserializing PatternBindingDecl #45in 'PureURLSession'
3. While deserializing 'parsedResponseHeader' (VarDecl Remove the FunctionEnumeration data structure from CalleeAnalysis #10)in 'PureURLSession'
4. While deserializing decl Remove extra / in git commands in README #22 (XREF)in 'PureURLSession'
5. Cross-reference to module 'PureURLSession'
   ... URLSessionTask
   ... in an extension in module 'PureURLSession'
   ... _ParsedResponseHeader
6. While loading members for extension of URLSessionTask in module 'PureURLSession'
7. While deserializing 'createTransferBodyDataDrain' (FuncDecl Fix typo in AllocBoxToStack.cpp #286)in 'PureURLSession'
8. While deserializing decl [SIL] Add test case for crash triggered in swift::Parser::parseSILGlobal() #690 (XREF)in 'PureURLSession'
9. Cross-reference to module 'PureURLSession'
   ... URLSessionTask
   ... in an extension in module 'PureURLSession'
   ... _TransferState
   ... _DataDrain
10. If you're seeing a crash here, check that your SDK and dependencies match the versions used to build 'PureURLSession'
    <unknown>:0: error: unable to execute command: Aborted
    <unknown>:0: error: merge-module command failed due to signal 6 (use -v to see invocation)
    <unknown>:0: error: build had 1 command failures
    error: exit(1): /home/rfdickerson/swift-toolchains/
    ```