Build the import lib with _ENFORCE_ONLY_CORE_HEADERS

[StephanTLavavej]

Related to (and inspired by) #2141.

This will need some MSVC-internal changes, but the impact is surprisingly small.

• In stl/CMakeLists.txt, we need to define _ENFORCE_ONLY_CORE_HEADERS when building the msvcp${D_SUFFIX}_implib_objects.
  • We don't want to modify the other place that the IMPLIB_SOURCES are mentioned - libcpmt${FLAVOR_SUFFIX} is the static lib.
• nothrow.cpp was including <new>, a non-core header, but it can just include <vcruntime_new.h> which is where nothrow_t is actually defined.
  • We can keep _STD_BEGIN/_STD_END if we include <yvals_core.h>.
  • I'm dropping the useless // define nothrow comment.
• locale0.cpp and <xfacet> are too difficult to untangle at this time. I'm just exempting them from this enforcement.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

I've resolved a trivial adjacent-add conflict in stl/src/locale0.cpp by taking #2141's comment before this PR's #undef.