Bug 1233732 - disable MacroAssembler.h macro magic for clang-cl; r=nbp

clang-cl attempts to emulate MSVC's handling of __VA_ARGS__, but doesn't
quite get it right:

https://llvm.org/bugs/show_bug.cgi?id=25875

As a result of this, compiling files that #include MacroAssembler.h with
clang-cl result in fallbacks to MSVC.  Since falling back to MSVC is
non-ideal (and also causes problems around things like linking function
template instantiations), let's disable MacroAssembler.h's macro magic
for the time being.  Ideally, the problem will get fixed upstream
soon (even though it looks somewhat complicated); in the meantime,
fixing this issue lets forward progress be made when compiling Gecko
with clang-cl.

js/src/jit/MacroAssembler.h

@@ -169,8 +169,15 @@
     DEFINED_ON_EXPAND_ARCH_RESULTS(                     \
       (MOZ_FOR_EACH(DEFINED_ON_FWDARCH, (), ArchList)))
 
+// clang-cl doesn't exactly follow MSVC's custom rules for handling
+// __VA_ARGS__ in macros (see LLVM PR 25875), so avoid using this macro
+// there.
+#if defined(_MSC_VER) && defined(__clang__)
+# define DEFINED_ON(...) /* nothing */
+#else
 # define DEFINED_ON(...)                                \
     DEFINED_ON_MAP_ON_ARCHS((none, __VA_ARGS__))
+#endif
 
 # define PER_ARCH DEFINED_ON(ALL_ARCH)
 # define PER_SHARED_ARCH DEFINED_ON(ALL_SHARED_ARCH)