[cxx-interop] Use macosx paths when searching modulemaps for macCatalyst (#76735)

The existing code will use paths for "iphoneos" because the triples for
macCatalyst are in the form "<arch>-apple-iosXX.yy-macabi", which the
code interprets as an iPhone triple. This seems to work in Xcode because
the "iphoneos" folder is always there. When building in open source,
this seems to work because it is also normal to have the iPhone SDK
configured. This is not the case if you want to build the macOS SDK in
isolation.

The code changes detect the macCatalyst environment from the triple and
use the "macosx" paths instead, otherwise it follows the same code path
as before. This matches many places in the CMakeLists.txt files in which
macCatalyst support pieces are only enabled when the SDK is macOS.

The modified function is also used for the libc modulemap of non-Darwin
platforms, but for those triples, the macCatalyst environment should not
be detected, so the libc modulemap should not be affected by this
change.

This is a followup to #76260 and #74994.

Author: drodriguez

lib/ClangImporter/ClangIncludePaths.cpp

@@ -30,7 +30,11 @@ static std::optional<Path> getActualModuleMapPath(
     StringRef name, SearchPathOptions &Opts, const llvm::Triple &triple,
     bool isArchSpecific,
     const llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &vfs) {
-  StringRef platform = swift::getPlatformNameForTriple(triple);
+  StringRef platform;
+  if (swift::tripleIsMacCatalystEnvironment(triple))
+    platform = "macosx";
+  else
+    platform = swift::getPlatformNameForTriple(triple);
   StringRef arch = swift::getMajorArchitectureName(triple);
 
   Path result;