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>

src/tools/clangbackend/ipcsource/sourcerange.cpp

@@ -39,6 +39,16 @@ SourceRange::SourceRange()
 {
 }
 
+bool SourceRange::isNull() const
+{
+    return clang_Range_isNull(cxSourceRange);
+}
+
+bool SourceRange::isValid() const
+{
+    return !isNull() && start().offset() < end().offset();
+}
+
 SourceLocation SourceRange::start() const
 {
     return SourceLocation(clang_getRangeStart(cxSourceRange));