[clang] [clang] Support fixed point types in C++ (PR #67750)

[via cfe-commits]

PiJoules wrote:

> This is introducing an extension from C into C++ -- was there an RFC or something discussed about whether we wish to support this in C++?

The original request for this I believe was on one of my earlier patches in 2018 for adding fixed point support in clang, but limiting it only to C because mangling for the types did not exist for the C++ ABI. Unfortunately it's been so long that phabricator doesn't seem to show this in my history. The earliest request for this was in https://github.com/itanium-cxx-abi/cxx-abi/issues/56 and this was mentioned in a status update for fixed point types in https://lists.llvm.org/pipermail/llvm-dev/2021-March/149216.html. We have some users who would like to see fixed point support in C++ without having to go through helper classes.

I think the revert was appropriate since fixed point types should only be opt-in and gated by the `-ffixed-point` flag, but it looks like I accidentally just kept it unconditionally on for C++. This shouldn't affect the MSVC headers otherwise.

https://github.com/llvm/llvm-project/pull/67750