GCC has supported C++11 ref-qualifiers since 4.8.1

This requires incorporating __GNUC_PATCHLEVEL__ into our prerequisite
check, and renaming our __GNUC_PREREQ to LLVM_GNUC_PREREQ, since it is
now functionally different.

Patch by Chilledheart!

Differential Revision: http://reviews.llvm.org/D5879

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220332 91177308-0d34-0410-b5e6-96231b3b80d8