[clang] [clang] check deduction consistency when partial ordering function templates (PR #100692)

[Luke Lau via cfe-commits]

lukel97 wrote:

I did a bit of grepping but I couldn't find any obvious macro. But I realised that gcc actually does accept the reproducer, when compiled with `-std=gnu++98`: https://compiler-explorer.com/z/zsGY6xbh5

This flag is set when compiling SPEC, and seems to be where clang differs. 

https://github.com/llvm/llvm-project/pull/100692