[libc++] Enable availability based on the compiler instead of __has_extension (#84065)

__has_extension(...) doesn't work as intended when -pedantic-errors is
used with Clang. With that flag, __has_extension(...) is equivalent to
__has_feature(...), which means that checks like

    __has_extension(pragma_clang_attribute_external_declaration)

will return 0. In turn, this has the effect of disabling availability
markup in libc++, which is undesirable.

rdar://124078119