Issue Description:

While doing work on ownership, I compiled a sibgen version of SwiftCore and noticed that sil-opt can not deserialize it due to I believe an error when deserializing a type.

We currently have tests that make sure that we can always deserialize the optimized standard library and the -Onone standard library, but we do not check if we can deserialize silgen.

We should create some sort of automation to check this and have someone investigate this.

I am merging in a commit that ensures that the targets for generating sib/sibgen files are created but not invoked by default. You should be able to use it. I would give you a hash, but it is currently merging. The PR is:

#6351

The repo hashes around where this was checked was:

clang 3180cd1 Merge remote-tracking branch 'origin/swift-3.1-branch' into stable
cmark 912d9d4 Merge pull request #1 from hughbe/patch-1
compiler-rt d235517 Merge remote-tracking branch 'origin/swift-3.1-branch' into stable
llbuild bf3b11a [Xcode] Disable a noisy warning.
lldb d93dd0f Adding a Swift specific fix that goes along with SVN revision 289233 where we may not always have a DeclVendor.
llvm 325df4d [ARM] Fix for 64-bit CAS expansion on ARM32 with -O0
swift 1b84d13 [semantic-sil] Reify the split in SILArgument in between function and block arguments via subclasses.
swift-corelibs-foundation 173e9ea Merge pull request #709 from phausler/cflite-import
swift-corelibs-libdispatch a055ddb Merge pull request #186 from dgrove-oss/SR-3002
swift-corelibs-xctest 1879b06 Merge pull request #181 from thaliproject/update-copyright
swift-integration-tests decef53 Merge pull request #13 from apple/remove-swift-update-test
swift-xcode-playground-support 6ee810b Resign ownership of PlaygroundLogger as I will be leaving Apple at the end of the month. Connor will be taking over.
swiftpm d683bca Merge pull request #841 from aciidb0mb3r/fix-xcode-test-popen