[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