Diagnostic for undefined assembler local symbols.

Re-apply 133010, with fixes for inline assembler.

Original commit message:
"When an assembler local symbol is used but not defined in a module, a
Darwin assembler wants to issue a diagnostic to that effect."

Added fix to only perform the check when finalizing, as otherwise we're not
done and undefined symbols may simply not have been encountered yet.

Passes "make check" and a self-host check on Darwin.

llvm-svn: 133071