[cmake] Check the compiler version first

Otherwise users get messages from CheckAtomic about missing libatomic
instead of a sensible message that says "use GCC 4.7 or newer".

I structured the change along the lines of HandleLLVMStdlib.cmake, so
that the standalone build of Clang still gets the compiler version
check.

Reviewers: beanz

Differential Revision: http://reviews.llvm.org/D17789

llvm-svn: 262491