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

azure-pipelines.yml

@@ -100,7 +100,7 @@ jobs:
         testResultsFiles: meson-test-run.xml
         testRunTitle: $(System.JobName)
 
-- job: msys2_mingw
+- job: msys2
   pool:
     vmImage: VS2017-Win2016
   strategy:
@@ -108,9 +108,15 @@ jobs:
       gccx86ninja:
         MSYSTEM: MINGW32
         MSYS2_ARCH: i686
+        compiler: gcc
       gccx64ninja:
         MSYSTEM: MINGW64
         MSYS2_ARCH: x86_64
+        compiler: gcc
+      clangx64ninja:
+        MSYSTEM: MINGW64
+        MSYS2_ARCH: x86_64
+        compiler: clang
   variables:
     MSYS2_ROOT: $(System.Workfolder)\msys64
   steps:
@@ -124,20 +130,23 @@ jobs:
       displayName: Update MSYS2
     - script: |
         set PATH=%MSYS2_ROOT%\usr\bin;%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem
+        if %compiler%==gcc ( set "TOOLCHAIN=mingw-w64-$(MSYS2_ARCH)-toolchain" ) else ( set "TOOLCHAIN=mingw-w64-$(MSYS2_ARCH)-clang" )
         %MSYS2_ROOT%\usr\bin\pacman --noconfirm --needed -S ^
         base-devel ^
         git ^
         mercurial ^
         mingw-w64-$(MSYS2_ARCH)-cmake ^
+        mingw-w64-$(MSYS2_ARCH)-pkg-config ^
         mingw-w64-$(MSYS2_ARCH)-python2 ^
         mingw-w64-$(MSYS2_ARCH)-python3 ^
         mingw-w64-$(MSYS2_ARCH)-python3-setuptools ^
-        mingw-w64-$(MSYS2_ARCH)-toolchain
+        %TOOLCHAIN%
       displayName: Install Dependencies
     - script: |
         set PATH=%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem
         %MSYS2_ROOT%\usr\bin\bash -lc "wget https://github.com/mesonbuild/cidata/raw/master/ninja.exe; mv ninja.exe /$MSYSTEM/bin"
         set PATHEXT=%PATHEXT%;.py
+        if %compiler%==clang ( set CC=clang && set CXX=clang++ )
         %MSYS2_ROOT%\usr\bin\bash -lc "MSYSTEM= python3 run_tests.py --backend=ninja"
       env:
         CHERE_INVOKING: yes