[LLD] [COFF] Handle undefined weak symbols in LTO (#70430)

When reading the bitcode input, undefined weak symbols will show up as
undefined symbols - which fails the early pass of checking for missing
symbols in symtab.reportUnresolvable(), before doing the actual LTO
compilation.

Mark such symbols as deferUndefined (added in
3785a413 /
https://reviews.llvm.org/D89004 for the -wrap option), to let them pass
through this LTO precheck. After the LTO compilation, the weak undefined
symbols will point towards an absolute null symbol as default.

Such weak undefined symbols are used for the TLS init function in the
Itanium C++ ABI, for TLS variables that potentially need to run a
constructor, when accessed across translation units.

This fixes https://github.com/llvm/llvm-project/issues/64513.