[COFF] Remove undefined behavior from ObjectFile::createWeakExternal

LLD type-punned an integral type and a pointer type using a pointer
field.  This is problematic because the pointer type has alignment
greater than some of the integral values.

This would be less problematic if a union was used but it turns out the
integral values are only present for a short, transient, amount of time.

Let's remove this undefined behavior by skipping the punning altogether
by storing the state in a separate memory location: a vector which
informs us which symbols to process for weak externs.

llvm-svn: 263918