[PDB] Simplify symbol handling code, NFC
- Make mergeSymbolRecords a method of PDBLinker to reduce the number of
parameters it needs.
- Remove a stale FIXME comment about error handling. We already drop
unknown symbol records, log them, and continue.
- Update a comment about why we're copying the symbol record. We do it
to realign the record. We can already mutate the symbol record memory,
it's memory allocated by relocateDebugChunk.
- Avoid the extra `CVSymbol NewSym` variable. We can mutate Sym in
place, which is best, since we're mutating the underlying record anyway.
llvm-svn: 346817