[libc++] Fix modules issues on OS X

First, fix a collision with the Point type from MacTypes.h, which was
reported on Slack, 2022-07-31: https://cpplang.slack.com/archives/C2X659D1B/p1659284691275889

Second, rename the meta:: namespace to types::. OSX's "/usr/include/ncurses.h"
defines a `meta` function, and is (for some reason) included in
"<SDK>/usr/include/module.modulemap", so that identifier is off-limits
for us to use in anything that compiles with -fmodules:

    libcxx/test/support/type_algorithms.h:16:11: error: redefinition of 'meta' as different kind of symbol
    namespace meta {
               ^
    <SDK>/usr/include/ncurses.h:603:28: note: previous definition is here
    extern NCURSES_EXPORT(int) meta (WINDOW *,bool);                        /* implemented */
                                ^

Finally, add a CI configuration for modules on OS X to make sure it
does not regress.

Differential Revision: https://reviews.llvm.org/D144915

libcxx/test/std/strings/basic.string/string.access/at.pass.cpp

@@ -76,7 +76,7 @@ struct TestCaller {
 };
 
 TEST_CONSTEXPR_CXX20 bool test() {
-  meta::for_each(meta::character_types(), TestCaller());
+  types::for_each(types::character_types(), TestCaller());
 
   return true;
 }