[clang-tidy] Add fix-it support to `llvmlibc-inline-function-decl`

This is very simplistic and could be more thorough by replacing
an existing `LIBC_INLINE` in the wrong location or a redunant
`inline` when inserting the right macro use.  But as is this
suffices to automatically apply fixes for most or all of the
instances in the libc tree today and get working results (despite
some superfluous `inline` keywords left behind).

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D157164