[Reland][ADT][ConcurrentHashTable] adapt ConcurrentHashTable and its users to...

[Reland][ADT][ConcurrentHashTable] adapt ConcurrentHashTable and its users to LLVM_ENABLE_THREADS=0 mode.

This patch hides thread specific handling under LLVM_ENABLE_THREADS guard.
It also removes usages of thread_local variables, since it has a weak
support on some platforms. Instead, the patch uses single mutex for locking
allocator. That may be replaced with more effective allocator later.
f.e. D142318

Differential Revision: https://reviews.llvm.org/D147649