Small cleanup: Don't duplicate default behavior.

std::unique_ptr is null initialized and reset default to null.

Thanks to David Blaikie for noticing.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@215560 91177308-0d34-0410-b5e6-96231b3b80d8
Revert what looks like an unintended change in r215557.

Should fix test ulibc driver tests.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@215561 91177308-0d34-0410-b5e6-96231b3b80d8