Fix evaluation of LLVM_DEFINITIONS

CMake variable LLVM_DEFINITIONS collects preprocessor definitions provided
for host compiler that builds llvm components. A function
add_llvm_definitions was introduced in AddLLVMDefinitions.cmake to keep
track of these definitions and was intended to be a replacement for CMake
command add_definitions. Actually in many cases add_definitions is still
used and the content of LLVM_DEFINITIONS is not actual now. On the other
hand the current version of CMake allows getting set of definitions in a
more convenient way. This fix implements evaluation of the variable by
reading corresponding cmake property.

Differential Revision: https://reviews.llvm.org/D31125

llvm-svn: 298336