[clang] always print clang errors on stderr

Summary:
clang errors would get diverted to log files instead of being printed on the
console. Also log other types of clang errors and warnings more often.

Before:

  $ infer -- clang -c nosuchfile.c
  $ # nothing gets printed, exit code set to 1

After:

  $ infer -- clang -c nosuchfile.c
  clang-4.0: error: no such file or directory: 'nosuchfile.c'

Also print more messages in case of compilation errors:

  $ echo ')' >> a.c
  $ infer -- clang -c a.c
  a.c:1:1: error: expected identifier or '('
  )
  ^
  1 error generated.
  Error: the following clang command did not run successfully:
    '/home/jul/infer/facebook-clang-plugins/clang/install/bin/clang-4.0' "@/home/jul/infer/examples/infer-out/clang/clang_command_2a0a84.txt"

(only the last 2 lines are new)

Reviewed By: akotulski

Differential Revision: D4183039

fbshipit-source-id: b9a9065

infer/src/clang/Capture.re

@@ -110,15 +110,21 @@ let run_and_validate_clang_frontend ast_source =>
     }
   };
 
-let run_clang clang_command read =>
+let run_clang clang_command read => {
+  let exit_with_error exit_code => {
+    Logging.stderr
+      "Error: the following clang command did not run successfully:@\n  %s@\n" clang_command;
+    exit exit_code
+  };
   /* NOTE: exceptions will propagate through without exiting here */
   switch (with_process_in clang_command read) {
   | (res, Unix.WEXITED 0) => res
   | (_, Unix.WEXITED n) =>
     /* exit with the same error code as clang in case of compilation failure */
-    exit n
+    exit_with_error n
-  | _ => exit 1
+  | _ => exit_with_error 1
-  };
+  }
+};
 
 let run_plugin_and_frontend source_path frontend clang_args => {
   let clang_command = ClangCommand.command_to_run (ClangCommand.with_plugin_args clang_args);

infer/src/clang/ClangWrapper.re

@@ -78,10 +78,10 @@ let exec_action_item =
   | ClangError error => {
       /* An error in the output of `clang -### ...`. Outputs the error and fail. This is because
          `clang -###` pretty much never fails, but warns of failures on stderr instead. */
-      Logging.err "%s" error;
+      Logging.stderr "%s@." error;
       exit 1
     }
-  | ClangWarning warning => Logging.err "%s@\n" warning
+  | ClangWarning warning => Logging.stderr "%s@\n" warning
   | Command clang_cmd => Capture.capture clang_cmd;
 
 let exe args xx_suffix => {