[COFF] Add MC support for emitting IMAGE_WEAK_EXTERN_ANTI_DEPENDENCY symbols

This is mostly useful for ARM64EC, which uses such symbols extensively.

One interesting quirk of ARM64EC is that we need to be able to emit weak
symbols that point at each other (so if either symbol is defined
elsewhere, both symbols point at the definition).  This required a few
changes to the way we handle weak symbols on Windows.

Differential Revision: https://reviews.llvm.org/D145208