Concurrency: avoid unnecessary dependency on LLVMSupport

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We do not have the llvm/Config/config.h header available in the forked
LLVMSupport library in our standard library packaging. This removes
that dependency by using the standard macro _POSIX_THREADS to detect
if we should use pthreads.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - d30e196

[compnerd]

@swift-ci please test macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - d30e196

[compnerd]

11:02:15 2022-01-07 11:02:11.048 xcodebuild[67400:68895479]  DVTDownloadable: Failed to get authorization with error Error Domain=DVTDownloadableErrors Code=3 "The authorization was denied since no user interaction was possible." UserInfo={AuthorizationErrorCode=-60007, NSLocalizedDescription=The authorization was denied since no user interaction was possible.}
11:02:15 
xcodebuild: error: The authorization was denied since no user interaction was possible.

Seems unrelated to the change?

[compnerd]

@swift-ci please test macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - d30e196

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13:57:00 Failed Tests (3):
13:57:00   Swift(watchsimulator-i386) :: Interop/Cxx/class/protocol-conformance-irgen.swift
13:57:00   Swift(watchsimulator-i386) :: Interop/Cxx/templates/class-template-instantiation.swift
13:57:00   Swift(watchsimulator-i386) :: Interop/Cxx/templates/class-template-uninstantiatable-members-irgen.swift

[compnerd]

@swift-ci please smoke test macOS platform