[clang] Use a StringRef instead of a raw char pointer to store builtin and call information

This avoids recomputing string length that is already known at compile time.

It has a slight impact on preprocessing / compile time, see

https://llvm-compile-time-tracker.com/compare.php?from=3f36d2d579d8b0e8824d9dd99bfa79f456858f88&to=e49640c507ddc6615b5e503144301c8e41f8f434&stat=instructions:u

This a recommit of e953ae5b and the subsequent fixes caa71355 and 06b90e2e.

The above patchset caused some version of GCC to take eons to compile clang/lib/Basic/Targets/AArch64.cpp, as spotted in aa171833.
The fix is to make BuiltinInfo tables a compilation unit static variable, instead of a private static variable.

Differential Revision: https://reviews.llvm.org/D139881