[ELF] - Print absolute file name in errors when possible.

[ELF] - Print absolute file name in errors when possible.

Currently LLD prints basename of source file name in error messages,
for example:
$ mkdir foo
$ echo 'void _start(void) { foobar(); }' > foo/bar.c
$ gcc -g -c foo/bar.c
$ bin/ld.lld -o out bar.o
bin/ld.lld: error: bar.c:1: undefined symbol 'foobar'
$
This should say:
bin/ld.lld: error: foo/bar.c:1: undefined symbol 'foobar'

This is PR31299

Differential revision: https://reviews.llvm.org/D27506

llvm-svn: 288966