Windows: Add support for building with clang.

This commit contains the minimum to make clang build/work with blender, asan and ninja build support is forthcoming

Things to note:

1) Builds and runs, and is able to pass all tests (except for the freestyle_stroke_material.blend test which was broken at that time for all platforms by the looks of it)

2) It's slightly faster than msvc when using cycles. (time in seconds, on an i7-3370)

victor_cpu
	msvc:3099.51
	clang:2796.43

pavillon_barcelona_cpu
	msvc:1872.05
	clang:1827.72

koro_cpu
	msvc:1097.58
	clang:1006.51

fishy_cat_cpu
	msvc:815.37
	clang:722.2

classroom_cpu
	msvc:1705.39
	clang:1575.43

bmw27_cpu
	msvc:552.38
	clang:561.53

barbershop_interior_cpu
	msvc:2134.93
	clang:1922.33

3) clang on windows uses a drop in replacement for the Microsoft cl.exe (takes some of the Microsoft parameters, but not all, and takes some of the clang parameters but not all) and uses ms headers + libraries + linker, so you still need visual studio installed and will use our existing vc14 svn libs.

4) X64 only currently, X86 builds but crashes on startup.

5) Tested with llvm/clang 6.0.0

6) Requires visual studio integration, available at https://github.com/LazyDodo/llvm-vs2017-integration

7) The Microsoft compiler spawns a few copies of cl in parallel to get faster build times, clang doesn't, so the build time is 3-4x slower than with msvc.

8) No openmp support yet. Have not looked at this much, the binary distribution of clang doesn't seem to include it on windows.

9) No ASAN support yet, some of the sanitizers can be made to work, but it was decided to leave support out of this commit.

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D3304

CMakeLists.txt

@@ -1612,7 +1612,12 @@ else()
 endif()
 
 # Visual Studio has all standards it supports available by default
-if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_C_COMPILER_ID MATCHES "Clang" OR CMAKE_C_COMPILER_ID MATCHES "Intel")
+# Clang on windows copies this behavior and does not support these switches
+if(
+	CMAKE_COMPILER_IS_GNUCC OR
+	(CMAKE_C_COMPILER_ID MATCHES "Clang" AND (NOT MSVC)) OR
+	(CMAKE_C_COMPILER_ID MATCHES "Intel")
+)
 	# Use C99 + GNU extensions, works with GCC, Clang, ICC
 	if(WITH_C11)
 		set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu11")