[Threading] Auto-detect threading in the preprocessor.
[Threading] Auto-detect threading in the preprocessor.

Some things (LLDB's language plugin, for instance), include Swift headers
from outside of our build system, and may not know to define one of the
threading package defines.

rdar://90776105

roambotics/swift,benlangmuir/swift,roambotics/swift,benlangmuir/swift,glessard/swift,JGiola/swift,roambotics/swift,glessard/swift,benlangmuir/swift,glessard/swift,roambotics/swift,JGiola/swift,benlangmuir/swift,JGiola/swift,benlangmuir/swift,JGiola/swift,JGiola/swift,apple/swift,apple/swift,glessard/swift,apple/swift,roambotics/swift,glessard/swift,apple/swift,roambotics/swift,apple/swift,glessard/swift,apple/swift,benlangmuir/swift,JGiola/swift