[ThinLTO][Legacy] Compute PreservedGUID based on IRName in Symtab

Instead of computing GUID based on some assumption about symbol mangling
rule from IRName to symbol name, lookup the IRName from all the symtabs
from all the input files to see if there are any matching symbols entry
provides the IRName for GUID computation.

rdar://65853754

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D84803