When defining the implicit move assignment operator, don't perform

When defining the implicit move assignment operator, don't perform
semantic analysis when taking the address of an xvalue. Instead, just
build the unary operator directly, since it's safe to do so (from the
IRgen and AST perspectives) for any glvalue. Fixes PR10822.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138935 91177308-0d34-0410-b5e6-96231b3b80d8