[clang-doc] Protect Index with mutex during reducing and generation stage

Idx in ClangDocContext instance was being modified by multiple threads
causing a seg fault.
A mutex is added to avoid this.

Differential Revision: https://reviews.llvm.org/D65915

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@368313 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed