[LLD] [COFF] Make weak aliases to implementations take priority over null pointers (#190491)

Normally, one uses weak aliases in one out of two ways.

Either one uses weak definitions to let a weak definition work as
fallback if a strong definition isn't available (which works with
link.exe as well), or as a sort of selectany COMDAT, to let multiple
weak definitions coexist, letting the linker pick any of them. (This
isn't supported by MS link.exe, but requires the LLD extension option
-lld-allow-duplicate-weak, normally implied by the mingw mode.)

Or, one uses weak references, to let a referencing translation unit
check at runtime, whether a symbol was found (at link time) or not,
optionally using the symbol.

In the latter case, the referencing object file provides a fallback
value for the weak symbol, as an absolute null symbol.

Previously, if we had multiple weak externals for the same symbol, we'd
pick any (the first one in practice); this is compatible with either of
the use cases above.

However if both use cases are combined, then we suddenly have multiple
conflicting weak externals for the same symbol; both one (or more)
symbols providing a fallback implementation for the symbol, and the
referencer that provides an absolute null pointer for the symbol.

In these cases, instead of just picking whichever choice the linker saw
first, prefer actual concrete implementations over the absolute null
symbol.

For mingw mode, this makes linking a translation unit referencing a
symbol declared with `__attribute__((weak))` work consistently if
linking it against another translation unit providing that symbol
defined with `__attribute__((weak))`, regardless of the order that those
two object files are linked.