[clang] [clang] Predefined macros for float128 support (PR #67196)

[Eric Christopher via cfe-commits]

https://github.com/echristo commented:

In general this looks good. Since we don't have full compiler-rt support for this yet we may want to wait until that to merge. Alex is already looking at this and coordinating with his needs is probably the best solution.

https://github.com/llvm/llvm-project/pull/67196