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
This patch is extracted from D96035. It adds StringPool class.
StringPool allows to store strings in parallel. It also allows
to have string data associated with the concrete string.
Reviewed By: JDevlieghere
Differential Revision: https://reviews.llvm.org/D140841
