enable MSA for clang

clang version 6.0.0 (trunk 310694) is able to compile MSA code.
Previous versions had an issue with _msa_fill_w(v32)
In this CL the macro DISABLE_CLANG_MSA is not set, allowing clang
to build the full MSA source.

TBR=kjellander@chromium.org
BUG=libyuv:715
TEST=gn gen out/Release "--args=is_debug=false target_os=\"android\" target_cpu=\"mips64el\" mips_arch_variant=\"r6\" mips_use_msa=true is_component_build=true is_clang=true"
ninja -v -C out/Release libyuv_unittest

Change-Id: I47401e3b1a3e4c57d9626ec2d3cd131c3ccf613c
Reviewed-on: https://chromium-review.googlesource.com/656501Reviewed-by: Frank Barchard <fbarchard@google.com>

include/libyuv/compare_row.h

@@ -43,8 +43,10 @@ extern "C" {
 #endif  // __clang__
 
 // clang 6 mips issue https://bugs.chromium.org/p/libyuv/issues/detail?id=715
+// broken in clang version 6.0.0 (trunk 308728)
+// fixed in clang version 6.0.0 (trunk 310694)
 #if defined(__clang__)
-#define DISABLE_CLANG_MSA 1
+// #define DISABLE_CLANG_MSA 1
 #endif
 
 #if !defined(LIBYUV_DISABLE_X86) && defined(_M_IX86) && \

include/libyuv/row.h

@@ -76,8 +76,10 @@ extern "C" {
 #endif  // VisualStudio >= 2012
 
 // clang 6 mips issue https://bugs.chromium.org/p/libyuv/issues/detail?id=715
+// broken in clang version 6.0.0 (trunk 308728)
+// fixed in clang version 6.0.0 (trunk 310694)
 #if defined(__clang__)
-#define DISABLE_CLANG_MSA 1
+// #define DISABLE_CLANG_MSA 1
 #endif
 
 // The following are available on all x86 platforms:

include/libyuv/scale_row.h

@@ -51,8 +51,10 @@ extern "C" {
 #endif  // VisualStudio >= 2012
 
 // clang 6 mips issue https://bugs.chromium.org/p/libyuv/issues/detail?id=715
+// broken in clang version 6.0.0 (trunk 308728)
+// fixed in clang version 6.0.0 (trunk 310694)
 #if defined(__clang__)
-#define DISABLE_CLANG_MSA 1
+// #define DISABLE_CLANG_MSA 1
 #endif
 
 // The following are available on all x86 platforms: