[find-all-symbols] Fix racy yaml file writing.

[find-all-symbols] Fix racy yaml file writing.

If multiple find-all-symbols processes access the temporary directory
simultaneously with two files with the same name they would collide and
create a broken yaml file. Fix this by using the safe createUniqueFile
API from LLVM instead.

Differential Revision: http://reviews.llvm.org/D19717

llvm-svn: 268021