[PATCH] CMake: add LLVM_INSTALL_TOOLCHAIN_ONLY option

[Hans Wennborg]

  With this patch, the Clang side patch from http://llvm-reviews.chandlerc.com/D1498 and a corresponding patch to lld, using this option will result in the following install dir:

  ./bin/clang++.exe
  ./bin/clang-check.exe
  ./bin/clang-cl.exe
  ./bin/clang-format.exe
  ./bin/clang.exe
  ./bin/git-clang-format
  ./bin/libclang.dll
  ./bin/lld.exe
  ./bin/llvm-ar.exe
  ./bin/llvm-objdump.exe
  ./include/clang-c/CXCompilationDatabase.h
  ./include/clang-c/CXString.h
  ./include/clang-c/Index.h
  ./include/clang-c/Platform.h
  ./lib/clang/3.4/include
  ./lib/clang/3.4/include/altivec.h
  ./lib/clang/3.4/include/ammintrin.h
  ./lib/clang/3.4/include/arm_neon.h
  ./lib/clang/3.4/include/avx2intrin.h
  ./lib/clang/3.4/include/avxintrin.h
  ./lib/clang/3.4/include/bmi2intrin.h
  ./lib/clang/3.4/include/bmiintrin.h
  ./lib/clang/3.4/include/cpuid.h
  ./lib/clang/3.4/include/emmintrin.h
  ./lib/clang/3.4/include/f16cintrin.h
  ./lib/clang/3.4/include/float.h
  ./lib/clang/3.4/include/fma4intrin.h
  ./lib/clang/3.4/include/fmaintrin.h
  ./lib/clang/3.4/include/immintrin.h
  ./lib/clang/3.4/include/iso646.h
  ./lib/clang/3.4/include/limits.h
  ./lib/clang/3.4/include/lzcntintrin.h
  ./lib/clang/3.4/include/mm3dnow.h
  ./lib/clang/3.4/include/mmintrin.h
  ./lib/clang/3.4/include/mm_malloc.h
  ./lib/clang/3.4/include/module.map
  ./lib/clang/3.4/include/nmmintrin.h
  ./lib/clang/3.4/include/pmmintrin.h
  ./lib/clang/3.4/include/popcntintrin.h
  ./lib/clang/3.4/include/prfchwintrin.h
  ./lib/clang/3.4/include/rdseedintrin.h
  ./lib/clang/3.4/include/rtmintrin.h
  ./lib/clang/3.4/include/smmintrin.h
  ./lib/clang/3.4/include/stdalign.h
  ./lib/clang/3.4/include/stdarg.h
  ./lib/clang/3.4/include/stdbool.h
  ./lib/clang/3.4/include/stddef.h
  ./lib/clang/3.4/include/stdint.h
  ./lib/clang/3.4/include/stdnoreturn.h
  ./lib/clang/3.4/include/tgmath.h
  ./lib/clang/3.4/include/tmmintrin.h
  ./lib/clang/3.4/include/unwind.h
  ./lib/clang/3.4/include/varargs.h
  ./lib/clang/3.4/include/wmmintrin.h
  ./lib/clang/3.4/include/x86intrin.h
  ./lib/clang/3.4/include/xmmintrin.h
  ./lib/clang/3.4/include/xopintrin.h
  ./lib/clang/3.4/include/__wmmintrin_aes.h
  ./lib/clang/3.4/include/__wmmintrin_pclmul.h
  ./lib/libclang.lib

http://llvm-reviews.chandlerc.com/D1428