[clang] [clang] Allow custom bug report messages (PR #157536)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Qualcomm currently has an internal patch that changes the string.  I think this proposal is sufficient for our usage.

Note that there are a couple of clang regression tests that check for the "PLEASE submit a bug report" text; any patch here also needs to update those tests.

https://github.com/llvm/llvm-project/pull/157536