Bug 1080968 - Don't use constexpr on clang-cl when emulating MSVC 2013

constexpr-ness of things can change the ABI requirements, and MSVC 2013
doesn't support constexpr, so choosing it for clang-cl when emulating
MSVC 2013 will cause ABI incomap issues between object files compiled
with the two compilers.

This reverts part of the commit for the original bug.

mfbt/Attributes.h

@@ -58,10 +58,7 @@
 #    define MOZ_HAVE_EXPLICIT_CONVERSION
 #  endif
 #  ifdef __clang__
-     /* clang-cl probably supports constexpr and explicit conversions. */
-#    if __has_extension(cxx_constexpr)
-#      define MOZ_HAVE_CXX11_CONSTEXPR
-#    endif
+     /* clang-cl probably supports explicit conversions. */
 #    if __has_extension(cxx_explicit_conversions)
 #      define MOZ_HAVE_EXPLICIT_CONVERSION
 #    endif