Parse the value symbol tables

[woodruffw]

Function mapping (#25) requires this, so I can't make meaningful progress on functions until this is done.

Just some notes to myself:

• parseValueSymbolTable in LLVM's bitcode reader
• MODULE_CODE_VSTOFFSET tells LLVM's bitcode reader where the module-level VST is, but we don't need that in our case (since we do eager parsing); however, we should still validate that there's exactly one MODULE_CODE_VSTOFFSET
• VST record codes:
  • VST_CODE_ENTRY
  • VST_CODE_FNENTRY
  • VST_CODE_BBENTRY
  • Others?

[woodruffw]

Also: module-level VSTs are parsed separately, via LLVM's parseGlobalValueSymbolTable. AFAICT, the only difference is that module-level VSTs can only contain VST_CODE_FNENTRY and none of the other codes.

Edit: More accurately, module-level VSTs are only parsed separately if there's also a module-level symbol table. When that table is present, VST_CODE_FNENTRY has the structure [valueid, offset] instead of [valueid, offset, namechar x N]. So these cases do probably need to be handled separately, at least at the parse level if not the model level.