[lld][COFF] Fix lld-link crash when several .obj files built with /Zi refer to...

[lld][COFF] Fix lld-link crash when several .obj files built with /Zi refer to a .pdb file that failed to load

This patch relaxes the constraints on the error message saved in PDBInputFile when failing to load a pdb file.

Storing an `Error` member infers that it must be accessed exactly once, which doesn't fit in several scenarios:
- If an invalid PDB file is provided as input file but never used, a loading error is created but never handled, causing an assert at shutdown.
- PDB file created using MSVC's `/Zi` option : The loading error message must be displayed once per obj file.

Also, the state of `PDBInputFile` was altered when reading (taking) the `Error` member, causing issues:
 - accessing it (taking the `Error`) makes the object look valid whereas it's not properly initialized
 - read vs write concurrency on a same `PDBInputFile` in the ghash parallel algorithm

The solution adopted here was to instead store an optional error string, and generate Error objects from it on demand.

Differential Revision: https://reviews.llvm.org/D140333