Skip test common/123 if masm not found

'test cases/common/123 llvm ir and assembly' requires ML/ML64 (masm) on
Windows.  If we are using clang-cl, that might not be available, so skip
test in that case.

test cases/common/123 llvm ir and assembly/meson.build

@@ -31,12 +31,15 @@ foreach lang : ['c', 'cpp']
   if cc.get_argument_syntax() == 'msvc'
     cl = cc.cmd_array()
     if cpu == 'x86'
-      ml = find_program('ml')
+      ml = find_program('ml', required: false)
     elif cpu == 'x86_64'
-      ml = find_program('ml64')
+      ml = find_program('ml64', required: false)
     else
       error('Unsupported cpu family: "' + cpu + '"')
     endif
+    if not ml.found()
+      error('MESON_SKIP_TEST: ML (masm) not found')
+    endif
     # Preprocess file (ml doesn't support pre-processing)
     preproc_name = lang + square_base + '.i'
     square_preproc = custom_target(lang + square_impl + 'preproc',