[REG->5.11] QtImage formats/WebP no longer compiles with clang-cl (LLVM 6)

c:\dev\5-clangcl-511\qt-clangcl-511\qtimageformats\src\plugins\imageformats\webp>n
..\..\..\3rdparty\libwebp\src\dsp\alpha_processing_sse41.c(52,26):  error: always_inline function '_mm_shuffle_epi8'
      requires target feature 'ssse3', but would be inlined into function 'ExtractAlpha_SSE41' that is compiled without
      support for 'ssse3'
      const __m128i b0 = _mm_shuffle_epi8(a0, kCstAlpha0);
                         ^
..\..\..\3rdparty\libwebp\src\dsp\alpha_processing_sse41.c(53,26):  error: always_inline function '_mm_shuffle_epi8'
      requires target feature 'ssse3', but would be inlined into function 'ExtractAlpha_SSE41' that is compiled without
      support for 'ssse3'
      const __m128i b1 = _mm_shuffle_epi8(a1, kCstAlpha1);
                         ^
..\..\..\3rdparty\libwebp\src\dsp\alpha_processing_sse41.c(54,26):  error: always_inline function '_mm_shuffle_epi8'
      requires target feature 'ssse3', but would be inlined into function 'ExtractAlpha_SSE41' that is compiled without
      support for 'ssse3'
      const __m128i b2 = _mm_shuffle_epi8(a2, kCstAlpha2);
                         ^
..\..\..\3rdparty\libwebp\src\dsp\alpha_processing_sse41.c(55,26):  error: always_inline function '_mm_shuffle_epi8'
      requires target feature 'ssse3', but would be inlined into function 'ExtractAlpha_SSE41' that is compiled without
      support for 'ssse3'
      const __m128i b3 = _mm_shuffle_epi8(a3, kCstAlpha3);
                         ^
4 errors generated.