[LLD][COFF] Cleanup error messages / add more coverage tests

[LLD][COFF] Cleanup error messages / add more coverage tests

- Log the reason for a PDB or precompiled-OBJ load failure
- Properly handle out-of-date PDB or precompiled-OBJ signature by displaying a corresponding error
- Slightly change behavior on PDB failure: any subsequent load attempt from another OBJ would result in the same error message being logged
- Slightly change behavior on PDB failure: retry with filename only if previous error was ENOENT ("no such file or directory")
- Tests: a. for native PDB errors; b. cover all the cases above

Differential Revision: https://reviews.llvm.org/D51559

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341825 91177308-0d34-0410-b5e6-96231b3b80d8