Avoid a static_assert in FxAllocAllocator in MSVC debug mode.

Small fix for when the GN enable_iterator_debugging variable is set to
true.

Bug: pdfium:1510
Change-Id: Ie83db1addc796ca07dcc8ff6654581cd05015f34
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/68630
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fxcrt/fx_memory_wrappers.h

diff --git a/core/fxcrt/fx_memory_wrappers.h b/core/fxcrt/fx_memory_wrappers.h
index 34ac63a..d7ccd89 100644
--- a/core/fxcrt/fx_memory_wrappers.h
+++ b/core/fxcrt/fx_memory_wrappers.h
@@ -23,8 +23,10 @@
 template <class T>
 struct FxAllocAllocator {
  public:
+#if !defined(COMPILER_MSVC) || defined(NDEBUG)
   static_assert(std::is_arithmetic<T>::value,
                 "Only numeric types allowed in this partition");
+#endif
 
   using value_type = T;
   using pointer = T*;