Make static counters in ASTContext non-static.

Summary:
Fixes a data race and makes it possible to run clang-based tools in
multithreaded environment with TSan.

Reviewers: ilya-biryukov, riccibruno

Reviewed By: riccibruno

Subscribers: riccibruno, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58612

llvm-svn: 354795