Clang: Filter out invalid diagnostic ranges

Apparently libclang might return invalid ranges.

Now we discard the invalid ranges. Since there is a diagnostic location
(in addition to ranges) the editor will still display an indication for
the user.

Task-number: QTCREATORBUG-15272
Change-Id: I351e136b9925a53fb2273a394e17873c5533798d
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

tests/unit/unittest/data/diagnostic_source_range.cpp

@@ -7,3 +7,7 @@ int function(XXX i)
 {
     i + 20;
 }
+
+struct Foo {
+    someIdentifierLeadingToInvalidRange;
+};