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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168993 91177308-0d34-0410-b5e6-96231b3b80d8