Revert "Change the timestamp of llvmcache-foo file to meet the thinLTO prune policy"

This reverts commit r345977.

A few bots failing because the invocation of "touch" is not accepted on
a couple other OSes. Specifically the -d argument is not accepted or requires
a different format.

http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/27103/steps/test_lld/logs/stdio
http://lab.llvm.org:8011/builders/lld-x86_64-freebsd/builds/24974/steps/test_lld/logs/stdio

llvm-svn: 345980