Use TempFile in lto caching.

This requires a small change to TempFile: allowing a discard after a
failed keep.

With this the cache now handles signals and reuses a fd instead of
reopening the file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@318322 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed