[clang] [CMake]Remove LLVM_USE_CRT* (PR #66850)

[Martin Storsjö via cfe-commits]

https://github.com/mstorsjo commented:

This looks mostly good to me, but a few cases might need setting `CMAKE_MSVC_RUNTIME_LIBRARY` instead of the removed variable.

The commit message needs improving though. This isn't related to the discussion about crt allocators at all.

Instead the commit message should explain that this setting is deprecated by the new CMake mechanism that we now prefer, mention that this was already marked as deprecated since July (referencing the commit c6bd873403a8ac6538a3fe3b3c2fe39c16b146a6), and mention that the current release branch, 17.x, still contains this option in deprecated form (so people upgrading through each major release won't have a silent break, but will get deprecation about using the old option form and have a chance to change it before it is removed).

https://github.com/llvm/llvm-project/pull/66850