Let llvm-cvtres (and lld-link) report duplicate resources

If two .res files contain the same resource, cvtres.exe (and hence
link.exe) reject the input with this message:

    CVTRES : fatal error CVT1100: duplicate resource.  type:STRING, name:101, language:0x0409
    LINK : fatal error LNK1123: failure during conversion to COFF: file invalid or corrupt

llvm-cvtres (and lld-link) used to silently pick one of the duplicate
resources instead. This patch makes them report an error as well.
We slightly improve on cvtres by printing the name of two .res files
containing duplicate entries as well.

Differential Revision: https://reviews.llvm.org/D61049

llvm-svn: 359083