[Clang] Avoid crashing when generating crash diagnostics when '#pragma clang...

[Clang] Avoid crashing when generating crash diagnostics when '#pragma clang __debug [assert|crash|parser_crash|llvm_fatal_error|llvm_unreachable|overflow_stack]' are used

Previously, when the above '#pragma clang __debug' were used, Driver::generateCompilationDiagnostics() wouldn't work as expected.
The 'clang -E' process created for diagnostics would crash, because it would reach again the intended crash in Pragma.cpp, PragmaDebugHandler::HandlePragma() while preprocessing.

When generating crash diagnostics, we now disable the intended crashing behavior with a new cc1 flag -disable-pragma-debug-crash.

Notes:
- #pragma clang __debug llvm_report_fatal isn't currently tested by crash-report.c, because it needs exit() to be handled differently in -fintegrated-cc1 mode. See https://reviews.llvm.org/D73742 for an upcoming fix.
- This is also needed to further validate that -MF is removed from the 'clang -E ' crash diagnostic cmd-line (currently not the case). See https://reviews.llvm.org/D74076 for an upcoming fix.

Differential Revision: https://reviews.llvm.org/D74070