Change OwningPtr::take() to OwningPtr::release().

This is a precursor to moving to std::unique_ptr.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203275 91177308-0d34-0410-b5e6-96231b3b80d8
Replace OwningPtr::isValid() with conversion to bool.

This is a precursor to moving to std::unique_ptr.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203277 91177308-0d34-0410-b5e6-96231b3b80d8