[clang] Add support for __builtin_memset_inline

In the same spirit as D73543 and in reply to https://reviews.llvm.org/D126768#3549920 this patch is adding support for `__builtin_memset_inline`.

The idea is to get support from the compiler to easily write efficient memory function implementations.

This patch could be split in two:
 - one for the LLVM part adding the `llvm.memset.inline.*` intrinsics.
 - and another one for the Clang part providing the instrinsic as a builtin.

Differential Revision: https://reviews.llvm.org/D126903