swift234

[cmake] Remove manual setting of include_directories for LLVM include dirs.

LLVM already provides these directories via the LLVMConfig.cmake variable
LLVM_INCLUDE_DIRS, which we are already using to import those directories.

Thus this work is and even worse gives the illusion that we are not delegating
the computation of LLVM's include paths to LLVM (which is really what we are
doing and should be doing).

rdar://26154980