1. Add an accessor function to MCSymbolizer to retrieve addresses
referenced by a symbolizable operand, but not resolved to a symbol.
That way, the caller can synthesize labels at those addresses and
then retry disassembling the section.
2. Implement that in AMDGPU -- a failed symbol lookup results in the
address being added to a vector returned by the new function.
3. Use that in llvm-objdump when using MCSymbolizer (which only happens
on AMDGPU) and SymbolizeOperands is on.
Differential Revision: https://reviews.llvm.org/D101145
Change-Id: I19087c3bbfece64bad5a56ee88bcc9110d83989e
8710eff6c3