add source information to diagnostic messages reported for failures

The rationale is to help simplify troubleshooting the errors reported
by compile-testing.

Author: pettermahlen

src/main/java/com/google/testing/compile/CompilationSubject.java

@@ -183,11 +183,18 @@ private void checkDiagnosticCount(
   }
 
   private static String messageListing(
-      Iterable<? extends Diagnostic<?>> diagnostics, String headingFormat, Object... formatArgs) {
+      Iterable<? extends Diagnostic<? extends JavaFileObject>> diagnostics,
+      String headingFormat,
+      Object... formatArgs) {
     StringBuilder listing =
         new StringBuilder(String.format(headingFormat, formatArgs)).append('\n');
-    for (Diagnostic<?> diagnostic : diagnostics) {
-      listing.append(diagnostic.getMessage(null)).append('\n');
+    for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics) {
+      listing.append(
+          String.format(
+              "%s: %d - %s\n",
+              diagnostic.getSource().getName(),
+              diagnostic.getLineNumber(),
+              diagnostic.getMessage(null)));
     }
     return listing.toString();
   }

src/test/java/com/google/testing/compile/CompilationSubjectTest.java

@@ -415,6 +415,21 @@ public void hadWarningCount() {
       assertThat(compilerWithWarning().compile(sourceFile)).hadWarningCount(2);
     }
 
+    @Test
+    public void hadWarningCountReportsLineNumbers() {
+      expectFailure
+          .whenTesting()
+          .about(compilations())
+          .that(compilerWithWarning().compile(sourceFile))
+          .hadWarningCount(0);
+      AssertionError expected = expectFailure.getFailure();
+
+      assertThat(expected.getMessage())
+          .contains(String.format("%s: 6 - this is a message", sourceFile.getName()));
+      assertThat(expected.getMessage())
+          .contains(String.format("%s: 7 - this is a message", sourceFile.getName()));
+    }
+
     @Test
     public void hadWarningCount_wrongCount() {
       expectFailure