[llvm-profdata] Revert r271709 and the 3 subsequent commits - the code

and/or tests aren't working on Windows currently.

There seems to be some problem with quoting the file paths. I don't
understand the test structure here or the code well enough to try to
come up with a way to correctly handle paths with back slashes in them,
and this has caused the Windows builds to be failing for 7 hours now, so
I'm reverting the whole thing to bring them back to life. Sorry for the
disruption, but a couple of these were bug fixes anyways that can be
folded into a fresh commit.

Reverts the following patches:

r271756: Clean up the way we create the input filenames buffer (NFC)
r271748: Fix use-after-free from discarded MemoryBuffer (NFC)
r271710: Fix option description (NFC)
r271709: Add option to ingest filepaths from a file
llvm-svn: 271760