[clang analysis] Make mutex guard detection more reliable.

-Wthread-safety was failing to detect certain AST patterns it should
detect. Make the pattern detection a bit more comprehensive.

Due to an unrelated bug involving template instantiation, this showed up
as a regression in 10.0 vs. 9.0 in the original bug report. The included
testcase fails on older versions of clang, though.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45323 .

Differential Revision: https://reviews.llvm.org/D76943