[libc++] Re-enable the clang_modules_include test for Objective-C++

This reverts commit aa60b26, which was a temporary workaround.

Author: ldionne

clang/foo

@@ -47,11 +47,8 @@
 #include <{header}>
 """)
 
-# TODO: Remove the UNSUPPORTED{BLOCKLIT}: clang-modules-build once issues with this test have been figured out.
 print(f"""\
 //--- __std_clang_module.compile.pass.mm
-// UNSUPPORTED{BLOCKLIT}: clang-modules-build
-
 // RUN{BLOCKLIT}: %{{cxx}} %s %{{flags}} %{{compile_flags}} -fmodules -fcxx-modules -fmodules-cache-path=%t -fsyntax-only
 
 // REQUIRES{BLOCKLIT}: clang-modules-build