Port r363962 to COFF: Deduplicate undefined symbol diagnostics

lld/coff already deduplicated undefined symbols on a TU level: It would
group all references to a symbol from a single TU. This makes it so that
references from all TUs to a single symbol are grouped together.

Since lld/coff almost did what I thought it did already, the patch is
much smaller than the elf version. The only not local change is that
getSymbolLocations() now returns a vector<string> instead of a string,
so that the undefined symbol reporting code can know how many references
to a symbol exist in a given TU.

Fixes PR42260 for lld/coff.

Differential Revision: https://reviews.llvm.org/D63646

llvm-svn: 364285