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

[Alexander Kornienko via cfe-commits]

alexfh wrote:

It was faster than I thought. The original code for case 2 is indeed invalid. Having looked carefully at the two definitions of PrintTo, I clearly see they are violating ODR. And both are originating in the internal code, so nothing specific to Eigen. Thanks again for the good explanation!

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