MissingSummary warning doesn't include filename

[pburka]

Description of the problem / feature request:

The MissingSummary warning doesn't include the name of the file that caused the warning.

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

$ cat foo/Foo.java
package foo;
/** */
public class Foo {}

$ javac -version -processorpath dataflow-shaded-3.1.2.jar:error_prone_core-2.4.0-with-dependencies.jar:jFormatString-3.0.0.jar:javac-9+181-r4173-1.jar:javacutil-2.5.7.jar:threeten-extra-1.5.0.jar -Xplugin:ErrorProne -XDcompilePolicy=byfile foo/Foo.java
javac 11.0.8
warning: [MissingSummary] A summary line is required on public/protected Javadocs.
    (see http://google.github.io/styleguide/javaguide.html#s7.2-summary-fragment)
1 warning

What version of Error Prone are you using?

2.4.0

Expected behavior

The warning should include the name of the file and the relevant line number, if any, just like other warnings from error-prone. E.g.:

foo/Foo.java:2: warning: [MissingSummary] ...

[vitalyd]

Seems the necessary condition to tickle this bug is a completely empty javadoc stub (i.e. /** */), as in @pburka's example, since most occurrences of this lint emit the file and line number.