[LLD][COFF] Move type merging structures out of PDB.cpp. NFC

Introduce a new TypeMerger class, out of some type-merge-specific structures from PDB.cpp

No changes intended / this is only moving code around.

This patch is step 3. in "Proposed commit strategy" in D59226

Differential Revision: https://reviews.llvm.org/D60070

llvm-svn: 357525