[LLD][COFF] Fix crash with /summary and PCH input files

Before this patch /summary was crashing with some .PCH.OBJ files, because tpiMap[srcIdx++] was reading at the wrong location. When the TpiSource depends on a .PCH.OBJ file, the types should be offset by the previously merged PCH.OBJ set of indices.

Differential Revision: https://reviews.llvm.org/D88678