azure: Add msys2 clang to test matrix
azure: Add msys2 clang to test matrix

The clang package depends on the gcc package, so we need to explicitly
select the compiler (as the priority order built into meson will prefer
gcc to clang)

v2:
ensure $(MSYS2-ARCH)-pkg-config is installed

jpakkane/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,QuLogic/meson,becm/meson,jpakkane/meson,becm/meson,QuLogic/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,becm/meson,pexip/meson,becm/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,becm/meson,QuLogic/meson,becm/meson,QuLogic/meson,becm/meson,mesonbuild/meson,pexip/meson,pexip/meson,becm/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,becm/meson,jpakkane/meson