[CMake] llvm_process_sources: Introduce a parameter, ADDITIONAL_HEADERS.

ADDITIONAL_HEADERS is intended to add header files for IDEs as hint.

For example:
  add_llvm_library(LLVMSupport
    Host.cpp
    ADDITIONAL_HEADERS
      Unix/Host.inc
      Windows/Host.inc
    )

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199639 91177308-0d34-0410-b5e6-96231b3b80d8