[llvm] [Support] Apply `constexpr` to `getTypeName` (PR #127893)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

Yes, it looks like older versions of gcc don't declare `__PRETTY_FUNCTION__` as `constexpr`: https://gcc.gnu.org/PR66639

9.1 seems to be the minimum version that's OK with this patch: https://godbolt.org/z/Y6j6hdeov. All supported clang/msvc versions seem to be OK.

https://github.com/llvm/llvm-project/pull/127893