gn build: Windows: use a more standard format for PDB filenames
gn build: Windows: use a more standard format for PDB filenames

The current build was producing names like llvm-undname.exe.pdb, which looks unusual to me at least. This switches them to the more common llvm-undname.pdb style.

Differential Revision: https://reviews.llvm.org/D57613



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@353099 91177308-0d34-0410-b5e6-96231b3b80d8

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