Issue Description:

Hi, i ran in to an issue with debugging stdlib changes and believe this is an existing issue from 2017. Swif.org/fourms post here : Swift forums post

samding01 (JIRA User) found the original issue here: Sam's Swift forums post

How i cloned:
branch: master
clone: git clone https://github.com/apple/swift.git
./swift/utils/update-checkout --clone
How i built:
terminal: utils/build-script --release --debug-swift --debug-swift-stdlib
Xcode: utils/build-script --debug --xcode

How i am testing:
Xcode: build and run
terminal: lldb attached to Ninja-RelWithDebInfoAssert+stdlib-DebugAssert/swift-macosx-x86_64/bin/swift

Expected results - able to investigate my local variables on stack frame

Actual - error: in auto-import:
failed to get module 'Builtin' from AST context

ocess 20684 stopped

• thread [lldb][NFC] Actually test which method we call in TestCallOverriddenMethod #2, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  frame #0: 0x000000010b01f657 libswiftCore.dylib`Array.stuffz.getter(self=) at Array.swift:328
  325 }
  326
  327 public var stuffz:String {
  -> 328 let x = "foobar"
  329 let y = "dem breakpoints"
  330 let combined = x + y
  331 return combined
  Target 0: (swift) stopped.
  (lldb) n
  Process 20684 stopped

• thread [lldb][NFC] Actually test which method we call in TestCallOverriddenMethod #2, queue = 'com.apple.main-thread', stop reason = step over
  frame #0: 0x000000010b01f67c libswiftCore.dylib`Array.stuffz.getter(self=) at Array.swift:329
  326
  327 public var stuffz:String { 328 let x = "foobar" -> 329 let y = "dem breakpoints" 330 let combined = x + y 331 return combined 332 }
  Target 0: (swift) stopped.
  (lldb) po x
  error: in auto-import:
  failed to get module 'Builtin' from AST context
  (lldb) s
  Process 20684 stopped

• thread [lldb][NFC] Actually test which method we call in TestCallOverriddenMethod #2, queue = 'com.apple.main-thread', stop reason = step in
  frame #0: 0x000000010b0178d8 libswiftCore.dylib`String.init(start=0x000000010b75dc1f "dem breakpoints", utf8CodeUnitCount=15, isASCII=1) at String.swift:513
  510 utf8CodeUnitCount: Builtin.Word,
  511 isASCII: Builtin.Int1
  512 ) {
  -> 513 let bufPtr = UnsafeBufferPointer(
  514 start: UnsafeRawPointer(start).assumingMemoryBound(to: UInt8.self),
  515 count: Int(utf8CodeUnitCount))
  516 if let smol = _SmallString(bufPtr) {
  Target 0: (swift) stopped.
  (lldb) s
  Process 20684 stopped

• thread [lldb][NFC] Actually test which method we call in TestCallOverriddenMethod #2, queue = 'com.apple.main-thread', stop reason = step in
  frame #0: 0x000000010b01b1ff libswiftCore.dylib`UnsafeBufferPointer.init(start=, count=15) at UnsafeBufferPointer.swift:892
  889 @inlinable // unsafe-performance
  890 public init(start: UnsafePointer?, count: Int) {
  891 _precondition(
  -> 892 count >= 0, "UnsafeBufferPointer with negative count")
  893 _precondition(
  894 count == 0 || start != nil,
  895 "UnsafeBufferPointer has a nil start and nonzero count")
  Target 0: (swift) stopped.
  (lldb) po count
  error: in auto-import:
  failed to get module 'Builtin' from AST context