[clang] [Coverage] Map regions from system headers (PR #76950)

[NAKAMURA Takumi via cfe-commits]

chapuni wrote:

I haven't had a reduced testcase for it, though.

I expect it reproducible with `-fcoverage-mcdc -mllvm -system-headers-coverage` to build `LLVMSupport`. `clangCodeGen` will crash. (`llvm-cov` crashes w/o `system-headers-coverage`)

https://github.com/llvm/llvm-project/pull/76950