Provide a default constructor for TimeValue. This was used, but only in
if-ed out code paths and on Windows. Hopefully restores the Windows
build. Thanks to Reid Kleckner for helping triage this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171568 91177308-0d34-0410-b5e6-96231b3b80d8