[LLD][COFF] More detailed information for /failifmismatch

When mismatched #pragma detect_mismatch declarations occur, now print the conflicting OBJs.

  lld-link: error: /failifmismatch: mismatch detected for 'TEST':
  >>> test.obj has value 1
  >>> test2.obj has value 2

Fixes PR38579

Differential Revision: https://reviews.llvm.org/D58910

llvm-svn: 355543