This is a more correct way of linking against a specific runtime
library with the GCC-like clang frontend.
This avoids having to pass -D_DEBUG (and passes flags like -D_DLL,
which we should be passing if linking against the dynamic CRT).
When -fms-runtime-lib= is specified, each compiled object file gets
embedded directives instructing the linker about what CRT it should
link against. The -nostdlib we pass to the compiler driver only
inhibits the libs that the compiler driver passes to the linker
Thus, this also avoids having to specify -lmsvcrt, -lmsvcrtd,
-llibcmt or -llibcmtd and -loldnames.
Based on a patch by Andrew Ng.
The -fms-runtime-lib= option was added in Clang 16, so this patch
can only be landed once libcxx's compiler requirement has been
bumped to Clang 16 (which is expected to be done sometime after
Clang 17 has been branched or released).