Changed createTemporaryFile without FD to actually create a file.

Summary:
This commit changes semantics of createUniqueFile and
createTemporaryFile variants that do not return file descriptors.
Previously they only checked if files exist, therefore being subject
to race conditions. Now they will create an empty file to avoid them.

Functions that do not create a file are now called
getPotentiallyUniqueTempFileName and getPotentiallyUniqueFileName.

Reviewers: klimek, bkramer, krasimir, JDevlieghere, espindola

Reviewed By: klimek

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36827

llvm-svn: 327851