[clang] [clang] Turn -Wenum-constexpr-conversion into a hard error (PR #102364)

[Sam James via cfe-commits]

https://github.com/thesamesam approved this pull request.

Thanks @AaronBallman!

Other than gdb (thank you Carlos for working on that), I'm only aware of two issues:
* qtlocation -> boost (https://bugs.gentoo.org/895516 -> https://bugreports.qt.io/browse/QTBUG-116652 -> https://github.com/boostorg/mpl/issues/69)
* pycuda (https://bugs.gentoo.org/924401 - not looked into this, it might have been fixed upstream in a newer version)

cc @llvm/clang-vendors

https://github.com/llvm/llvm-project/pull/102364