Make clang-tidy fail when it ought to.

clang-tidy exits with an error code of 0 if the code compiles, even if
there are warnings.  Make it generate a file with the errors and fail
the test if this file exists.

Also remove the -fix flag - we don't want clang-tidy to fix things in CI
(yet?).

azure-pipelines.yml

@@ -217,6 +217,7 @@ phases:
     displayName: 'Clang-Format'
 
   - script: |
-      clang-tidy-8  src/override/malloc.cc  -header-filter="`pwd`/*"  -fix -- -std=c++17 -mcx16
+      clang-tidy-8  src/override/malloc.cc  -header-filter="`pwd`/*" -export-fixes=tidy.fail -- -std=c++17 -mcx16
+      if [ -f tidy.fail ] ; then exit 1 ; fi
 
     displayName: 'Clang-Tidy'