[clang] [clang][test] Make libclang CrashTests use threadsafe gtest death-test style (PR #196148)

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

> There are a bunch of other clang unittests that have EXPECT_DEATH; should we be doing this in all of them? I'm not clear why these two libclang tests are special here.

It depends on if the test will have `SignpostEmitter` triggered. PR is based on the actual failures saw in the CI, which I queried more than 50 occurrences. I don't see other significant failures.

https://github.com/llvm/llvm-project/pull/196148