Do not add -gsplit-dwarf to LLVM_DEFINITIONS.

This would cause the flag to appear in the output of "llvm-config --cppflags",
which should contain only preprocessor flags. The -gsplit-dwarf flag in
particular can cause problems with certain downstream users such as cgo.

Differential Revision: http://reviews.llvm.org/D5895

llvm-svn: 220410