cgo: add tests for errors

This commit adds tests for CGo preprocessing. There are various errors
that can be reported while preprocessing, and they should integrate well
with the compiler (including accurate source location tracking).

Also allow CGo preprocessing to continue after Clang encountered an
error, for a better view of what happened.

cgo/libclang.go

@@ -132,7 +132,6 @@ func (p *cgoPackage) parseFragment(fragment string, cflags []string, posFilename
 				addDiagnostic(C.clang_getDiagnosticInSet(diagnostics, C.uint(j)))
 			}
 		}
-		return
 	}
 
 	ref := storedRefs.Put(p)