loplugin:oncevar: empty strings

...which showed that checking the parent statement (which may be too large to)
in OnceVar::VisitDeclRefExpr is inadequate.

Change-Id: I07fb8ba9e2dfbd0c65a2723737d14abcddcefec4
Reviewed-on: https://gerrit.libreoffice.org/39757Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/check.cxx

@@ -14,6 +14,14 @@
 
 namespace loplugin {
 
+TypeCheck TypeCheck::NonConst() const {
+    return !type_.isNull() && !type_.isConstQualified()
+        ? *this : TypeCheck();
+        // returning TypeCheck(type_.getUnqualifiedType()) instead of *this
+        // may look tempting, but could remove sugar we might be interested in
+        // checking for
+}
+
 TypeCheck TypeCheck::NonConstVolatile() const {
     return
         (!type_.isNull() && !type_.isConstQualified()

compilerplugins/clang/check.hxx

@@ -37,6 +37,8 @@ public:
 
     explicit operator bool() const { return !type_.isNull(); }
 
+    TypeCheck NonConst() const;
+
     TypeCheck NonConstVolatile() const;
 
     TypeCheck Const() const;