clang as of Visual Studio 2019 doesn't support the __umulh intrinsic yet
VS2022 (at last version 17.10) does, so instead of a !defined(__clang__)
use a compiler version check to force the fallback for older versions.
See https://learn.microsoft.com/en-us/cpp/overview/compiler-versions for
a mapping of VisualStudio version to compiler version macro
diff -ur pdfium.org/third_party/fast_float/src/include/fast_float/float_common.h pdfium/third_party/fast_float/src/include/fast_float/float_common.h
--- pdfium.org/third_party/fast_float/src/include/fast_float/float_common.h	2025-03-14 14:19:56
+++ pdfium/third_party/fast_float/src/include/fast_float/float_common.h	2025-03-14 14:42:05
@@ -338,7 +338,7 @@
     return answer;
   }
   value128 answer;
-#if defined(_M_ARM64) && !defined(__MINGW32__)
+#if defined(_M_ARM64) && !defined(__MINGW32__) && defined(_MSC_VER) && _MSC_VER >= 1930
   // ARM64 has native support for 64-bit multiplications, no need to emulate
   // But MinGW on ARM64 doesn't have native support for 64-bit multiplications
   answer.high = __umulh(a, b);