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

intern/atomic/intern/atomic_ops_msvc.h

@@ -40,7 +40,11 @@
 #include <windows.h>
 #include <intrin.h>
 
-/******************************************************************************/
+#if defined (__clang__)
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
+#endif
+
 /* 64-bit operations. */
 #if (LG_SIZEOF_PTR == 8 || LG_SIZEOF_INT == 8)
 /* Unsigned */
@@ -205,4 +209,9 @@ ATOMIC_INLINE int8_t atomic_fetch_and_or_int8(int8_t *p, int8_t b)
 #endif
 }
 
+
+#if defined (__clang__)
+#  pragma GCC diagnostic pop
+#endif
+
 #endif /* __ATOMIC_OPS_MSVC_H__ */

intern/cycles/CMakeLists.txt

@@ -31,7 +31,7 @@ elseif(NOT WITH_CPU_SSE)
 	set(CXX_HAS_SSE FALSE)
 	set(CXX_HAS_AVX FALSE)
 	set(CXX_HAS_AVX2 FALSE)
-elseif(WIN32 AND MSVC)
+elseif(WIN32 AND MSVC AND NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
 	set(CXX_HAS_SSE TRUE)
 	set(CXX_HAS_AVX TRUE)
 	set(CXX_HAS_AVX2 TRUE)
@@ -306,7 +306,7 @@ if(WITH_CYCLES_CUDA_BINARIES AND (NOT WITH_CYCLES_CUBIN_COMPILER))
 		elseif(${CUDA_VERSION} EQUAL "9.1")
 			set(MAX_MSVC 1911)
 		endif()
-		if(NOT MSVC_VERSION LESS ${MAX_MSVC})
+		if(NOT MSVC_VERSION LESS ${MAX_MSVC} OR CMAKE_C_COMPILER_ID MATCHES "Clang")
 			message(STATUS "nvcc not supported for this compiler version, using cycles_cubin_cc instead.")
 			set(WITH_CYCLES_CUBIN_COMPILER ON)
 		endif()

intern/cycles/kernel/kernels/cpu/kernel_cpu_image.h

@@ -349,7 +349,7 @@ template<typename T> struct TextureInterpolator  {
 	 * Only happens for AVX2 kernel and global __KERNEL_SSE__ vectorization
 	 * enabled.
 	 */
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 	static ccl_always_inline
 #else
 	static ccl_never_inline