Update compatibility page to list some GCC language extensions that Clang does
not support as a possible reason for choosing GCC instead of Clang (and vice
versa). Weaken some of the claimed advantages of Clang in light of GCC
improvements.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196758 91177308-0d34-0410-b5e6-96231b3b80d8