Reuse temporary files for print-changed=diff

Summary:
Make the file name and descriptors static so that they are reused by
print-changed=diff. This avoids errors about being unable to create
temporary files when doing the later comparisons in a large compile.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: aeubanks (Arthur Eubanks)
Differential Revision: https://reviews.llvm.org/D100116