[ci/msys2] Add a job building in the clang/llvm environment

Clang is more strict in certain areas and will fail, like
https://github.com/harfbuzz/harfbuzz/pull/5079#issuecomment-2673805310
https://github.com/harfbuzz/harfbuzz/issues/5197

* Add CLANG64 to the matrix
* Install the virtual "cc" package which pulls in the default compiler
  for the respective environment
* Remove gcc-libs, it should be pulled in automatically
* Depend on MSYSTEM_PREFIX for deleting files to be environment agnostic

.github/workflows/msys2-ci.yml

@@ -21,6 +21,8 @@ jobs:
             MSYS2_ARCH: i686
           - MSYSTEM: MINGW64
             MSYS2_ARCH: x86_64
+          - MSYSTEM: CLANG64
+            MSYS2_ARCH: clang-x86_64
     name: ${{ matrix.MSYSTEM }}
 
     env:
@@ -42,8 +44,7 @@ jobs:
         install: >-
           mingw-w64-${{ matrix.MSYS2_ARCH }}-cairo
           mingw-w64-${{ matrix.MSYS2_ARCH }}-freetype
-          mingw-w64-${{ matrix.MSYS2_ARCH }}-gcc
-          mingw-w64-${{ matrix.MSYS2_ARCH }}-gcc-libs
+          mingw-w64-${{ matrix.MSYS2_ARCH }}-cc
           mingw-w64-${{ matrix.MSYS2_ARCH }}-gettext
           mingw-w64-${{ matrix.MSYS2_ARCH }}-glib2
           mingw-w64-${{ matrix.MSYS2_ARCH }}-gobject-introspection
@@ -56,7 +57,7 @@ jobs:
           mingw-w64-${{ matrix.MSYS2_ARCH }}-python-pip
     - name: Remove installed HarfBuzz DLLs
       run: |
-        rm -f -v /ming*/bin/libharfbuzz-*.dll
+        rm -f -v "$MSYSTEM_PREFIX"/bin/libharfbuzz-*.dll
     - name: Install Python Dependencies
       run: |
         pip3 install -r .ci/requirements-fonttools.txt --require-hashes