[ThinLTO] Move loading of cache entry to client

Summary:
Have the cache pass back the path to the cache entry when it
is ready to be loaded, instead of a buffer.

For gold-plugin we can simply pass this file back to gold directly,
which avoids expensive writing of a separate tmp file. Ensure
the cache entry is not deleted on cleanup by adjusting the setting
of the IsTemporary flags.

Moved the loading of the buffer into llvm-lto2 to maintain current
behavior.

Reviewers: mehdi_amini

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23946

llvm-svn: 279883