Stop pulling sources from compiler into runtimes

[etcwilde]

Description

Several libraries in the Swift runtimes pull sources from the compiler into the runtime libraries.
This is bad since we want to carefully control what is pulled into the runtime libraries for ABI and API purposes.

Libraries pulling headers from the compiler:

• SwiftStdlibStubs
• runtime
• Demangling
• CommandLineSupport
• Threadings

Libraries pulling sources from the compiler:

• Demangling
• Threading
• runtime (ThreadSanitizer.cpp)

These are the bits just going into swiftCore that I've noticed crossing the boundary.

Additional information

No response

[etcwilde]

The ELF swiftrt.o also pulls headers from the compiler source directory.