[LLD] [COFF] Try to report source locations for duplicate symbols

This fixes the second part of PR42407.

For files with dwarf debug info, it manually loads and iterates
.debug_info to find the declared location of variables, to allow
reporting them. (This matches the corresponding code in the ELF
linker.)

For functions, it uses the existing getFileLineDwarf which uses
LLVMSymbolizer for translating addresses to file lines.

In object files with codeview debug info, only the source location
of duplicate functions is printed. (And even there, only for the
first input file. The getFileLineCodeView function requires the
object file to be fully loaded and initialized to properly resolve
source locations, but duplicate symbols are reported at a stage when
the second object file isn't fully loaded yet.)

Differential Revision: https://reviews.llvm.org/D68975

llvm-svn: 375218