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>

etc/spack/defaults/darwin/packages.yaml

@@ -15,7 +15,11 @@
 # -------------------------------------------------------------------------
 packages:
   all:
-    compiler: [clang, gcc, intel]
+    compiler:
+    - apple-clang
+    - clang
+    - gcc
+    - intel
     providers:
       elf: [libelf]
       unwind: [apple-libunwind]