Threading library munges all backends into a single build

[etcwilde]

Description

The Threading runtime library currently builds all of the threading runtimes into a single library, using macros inside of the files to mask out the contents of the source file resulting in the library containing several empty objects and a single object with actual content. While it "works", this is kind of gross. It would be better if the build selected the appropriate file and only built the one file.

swift/stdlib/public/Threading/CMakeLists.txt

Lines 4 to 13 in 3f8ffaa

	list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/../../cmake/modules")
	include(AddSwiftStdlib)
	# This should *not* include ThreadSanitizer.cpp, as that is part of libswiftCore
	add_swift_target_library(swiftThreading OBJECT_LIBRARY
	"${SWIFT_SOURCE_DIR}/lib/Threading/C11.cpp"
	"${SWIFT_SOURCE_DIR}/lib/Threading/Linux.cpp"
	"${SWIFT_SOURCE_DIR}/lib/Threading/Pthreads.cpp"
	"${SWIFT_SOURCE_DIR}/lib/Threading/Win32.cpp"
	INSTALL_IN_COMPONENT never_install)

Additional information

No response

[etcwilde]

swiftCore also does similar things for Vector and CommandLine support.