Reapply r343058 with a fix for -DLLVM_ENABLE_THREADS=OFF.

Modifies lit to add a 'thread_support' feature that can be used in lit test
REQUIRES clauses. The thread_support flag is set if -DLLVM_ENABLE_THREADS=ON
and unset if -DLLVM_ENABLE_THREADS=OFF. The lit flag is used to disable the
multiple-compile-threads-basic.ll testcase when threading is disabled.

llvm-svn: 343122