Call clang-tidy based on WITH_TIDY* variables.

* Call clang-tidy before every C/C++ compilation if
  (1) clang-tidy is found at $(PATH_TO_CLANG_TIDY)
  (2) $(my_clang) is true
  (3) LOCAL_TIDY is 1 or true, or
      LOCAL_TIDY is undefined and WITH_TIDY is 1 or true.
* clang-tidy is called with -checks=$(my_tidy_checks),
  which has default '-*,google*,-google-readability*'
  and can be overwritten by WITH_TIDY_CHECKS.
* LOCAL_TIDY_CHECKS is appended to $(my_tidy_checks)
* Extra flags are passed to clang-tidy through
  WITH_TIDY_FLAGS or LOCAL_TIDY_FLAGS.
* To quickly find and fix clang-tidy warnings, set $(WITH_TIDY_ONLY)
  to 1 or true to skip compilation of C/C++ files.
* Add a PHONY tidy_only target,
  which includes all $(cpp_objects) and $(c_objects).
* The 'mm' and 'mmm' functions are changed to call make with
  the 'tidy_only' target when WITH_TIDY_ONLY is true or 1.
  In that case, only clang-tidy is called for C and C++ files.

Bug: http://b/27779618
Change-Id: I8adcfff217d68af49849b79aacee7d7654cafb1b

envsetup.sh

@@ -794,6 +794,9 @@ function mm()
               MODULES=all_modules
               ARGS=$@
             fi
+            if [ "1" = "${WITH_TIDY_ONLY}" -o "true" = "${WITH_TIDY_ONLY}" ]; then
+              MODULES=tidy_only
+            fi
             ONE_SHOT_MAKEFILE=$M $DRV make -C $T -f build/core/main.mk $MODULES $ARGS
         fi
     fi
@@ -845,6 +848,9 @@ function mmm()
           ARGS=$GET_INSTALL_PATH
           MODULES=
         fi
+        if [ "1" = "${WITH_TIDY_ONLY}" -o "true" = "${WITH_TIDY_ONLY}" ]; then
+          MODULES=tidy_only
+        fi
         ONE_SHOT_MAKEFILE="$MAKEFILE" $DRV make -C $T -f build/core/main.mk $DASH_ARGS $MODULES $ARGS
     else
         echo "Couldn't locate the top of the tree.  Try setting TOP."