[LLD][COFF] Convert file name to lowercase when inserting it into visitedLibs

It seems to be a bug in `LinkerDriver::findFile`, the file name is not converted
to lowercase when being inserted into `visitedLibs`. This is the only exception
in the file and all other places always convert file names to lowercase when
inserting them into `visitedLibs` (or `visitedFiles`).

Reviewed By: thieta, hans

Differential Revision: https://reviews.llvm.org/D127709