Separate out the tests for whether the compiler suports R-value

references from whether it supports an R-value reference *this. No
version of GCC today supports the latter, which breaks GCC C++11
compiles of LLVM and Clang now.

Also add doxygen comments clarifying what's going on here, and update
the usage in Optional. I'll update the usages in Clang next.

llvm-svn: 168993