Compute A-B when A or B is weak.

Compute A-B when A or B is weak.

Similar to r235222, but for the weak symbol case.

In an "ideal" assembler/object format an expression would always refer to the
final value and A-B would only be computed from a section in the same
comdat as A and B with A and B strong.

Unfortunately that is not the case with debug info on ELF, so we need an
heuristic.  Since we need an heuristic, we may as well use the same one as
gas:

* call weak_sym : produces a relocation, even if in the same section.
* A - weak_sym and weak_sym -A: don't produce a relocation if we can
  compute it.

This fixes pr23272 and changes the fix of pr22815 to match what gas does.

llvm-svn: 235227