[CMake] Move the setting of LLVM_COMPILER_IS_GCC_COMPATIBLE to a separate file

Currently LLVM_COMPILER_IS_GCC_COMPATIBLE is set as a side-effect of determining
the stdlib to use in HandleLLVMStdlib, which causes problems when attempting to
use AddLLVM from an installed LLVM toolchain, as HandleLLVMStdlib is not used.
Move the setting of this variable into DetermineGCCCompatible and include that
from both AddLLVM and HandleLLVMStdlib.

Differential Revision: http://reviews.llvm.org/D13216

llvm-svn: 248798