[ELF] Fail the link early if an output path is invalid

Patch from James Henderson.

If a user has a long link, e.g. due to a large LTO link, they do not
wish to run it and find that it failed because there was a mistake in
their command-line, after they waited for some significant amount of
time. This change adds some basic checking of the linker output file
path, which is run shortly after parsing the command-line and linker
script. An error is emitted if LLD cannot write to the specified path.

Differential Revision: https://reviews.llvm.org/D30449

llvm-svn: 297645