[Support] Change fatal_error_handler_t to take a const char* instead of std::string

https://commondatastorage.googleapis.com/chromium-browser-clang/llvm-include-analysis.html

Excessive use of the <string> header has a massive impact on compile time; its most commonly included via the ErrorHandling.h header, which has to be included in many key headers, impacting many source files that have no need for std::string.

As an initial step toward removing the <string> include from ErrorHandling.h, this patch proposes to update the fatal_error_handler_t handler to just take a raw const char* instead.

The next step will be to remove the report_fatal_error std::string variant, which will involve a lot of cleanup and better use of Twine/StringRef.

Differential Revision: https://reviews.llvm.org/D111049