[Foundation] Adjust Foundation shims for compatibility

[lorentey]

• Adjust stray public import of _SwiftFoundationOverlayShims
• Re-add Foundation shims that were removed in [Foundation] Import shims with @implementationOnly #28918. The current version of the Foundation overlay doesn’t use these, but we should still keep them in case a toolchain snapshot build picks up an overlay module from one of the SDKs in Xcode 11.

rdar://62339802

[lorentey]

@swift-ci test

[lorentey]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 057b27b

[lorentey]

19:45:39 ******************** TEST 'lldb-shell :: SwiftREPL/OpenClass.test' FAILED ********************
19:45:39 Script:
19:45:39 --
19:45:39 : 'RUN: at line 1';   /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/buildbot_incremental/lldb-macosx-x86_64/bin/lldb --no-lldbinit -S /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/buildbot_incremental/lldb-macosx-x86_64/test/Shell/lit-lldb-init --repl < /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/llvm-project/lldb/test/Shell/SwiftREPL/OpenClass.test 2>&1 | /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/buildbot_incremental/llvm-macosx-x86_64/bin/FileCheck /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/llvm-project/lldb/test/Shell/SwiftREPL/OpenClass.test
19:45:39 --
19:45:39 Exit Code: 1
19:45:39 
19:45:39 Command Output (stderr):
19:45:39 --
19:45:39 
/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/llvm-project/lldb/test/Shell/SwiftREPL/OpenClass.test:12:11: error: CHECK: expected string not found in input
19:45:39 // CHECK: $R0: Foo.X = 0
19:45:39           ^
19:45:39 <stdin>:1:1: note: scanning from here
19:45:39 (lldb) command source -s 0 '/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/buildbot_incremental/lldb-macosx-x86_64/test/Shell/lit-lldb-init'
19:45:39 ^
19:45:39 <stdin>:14:134: note: possible intended match here
19:45:39 5 LLDB 0x000000011473cd02 lldb_private::SwiftASTContext::CreateInstance(lldb::LanguageType, lldb_private::Module&, lldb_private::Target*, bool) + 7538
19:45:39                                                                                                                                      ^
19:45:39 
19:45:39 --
19:45:39 
19:45:39 ********************

[lorentey]

@swift-ci test macOS platform