Separate Apple Clang from LLVM Clang (#17110)

* Separate Apple Clang from LLVM Clang

Apple Clang is a compiler of its own. All places
referring to "-apple" suffix have been updated.

* Hack to use a dash in 'apple-clang'

To be able to use autodoc from Sphinx we need
a valid Python name for the module that contains
Apple's Clang code.

* Updated packages to account for the existence of apple-clang

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

* Added unit test for XCode related functions

Co-authored-by: Gregory Becker <becker33@llnl.gov>
Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

var/spack/repos/builtin/packages/dealii/package.py

@@ -301,7 +301,7 @@ def cmake_args(self):
             cxx_flags_release.extend(['-O3'])
         elif spec.satisfies('%intel'):
             cxx_flags_release.extend(['-O3'])
-        elif spec.satisfies('%clang'):
+        elif spec.satisfies('%clang') or spec.satisfies('%apple-clang'):
             cxx_flags_release.extend(['-O3', '-ffp-contract=fast'])
 
         # Python bindings