Better heuristic to only look through implicit copy/move ctors

At least recent libc++ has a std::string ctor overload without a (defaulted)
Allocator argument (which otherwise causes creation of a temporary Allocator
object and thus a ExprWithCleanups), so in C++17 mode (i.e., with no implicit
move CXXConstructExpr -> MaterializeTemporaryExpr -> CXXBindTemporaryExpr chain
in the way) CellInfo::toString (sw/source/filter/ww8/WW8TableInfo.cxx) has a
ReturnStmt of just

> ReturnStmt
> `-ImplicitCastExpr 'std::string':'class std::__1::basic_string<char>' <ConstructorConversion>
>   `-CXXConstructExpr 'std::string':'class std::__1::basic_string<char>' 'void (const char *)'
>     `-ImplicitCastExpr 'const char *' <NoOp>
>       `-ImplicitCastExpr 'char *' <ArrayToPointerDecay>
>         `-DeclRefExpr 'char [256]' lvalue Var 'sBuffer' 'char [256]'

that erroneously triggered loplugin:passstuffbyref.

Change-Id: I53c8911cb1356560692c003808280a103c399e25
Reviewed-on: https://gerrit.libreoffice.org/45916Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/passstuffbyref.cxx

@@ -310,8 +310,11 @@ bool PassStuffByRef::isReturnExprDisqualified(const Expr* expr)
     if (isa<ExprWithCleanups>(expr)) {
         return true;
     }
-    if (const CXXConstructExpr* constructExpr = dyn_cast<CXXConstructExpr>(expr)) {
-        if (constructExpr->getNumArgs()==1) {
+    if (const CXXConstructExpr* constructExpr = dyn_cast<CXXConstructExpr>(expr))
+    {
+        if (constructExpr->getNumArgs()==1
+            && constructExpr->getConstructor()->isCopyOrMoveConstructor())
+        {
             expr = constructExpr->getArg(0)->IgnoreParenCasts();
         }
     }

compilerplugins/clang/test/passstuffbyref.cxx

@@ -26,6 +26,13 @@ void f()
     s = new S(v1, v2);
 }
 
+struct S2 { S2(int); };
+
+S2 f2() {
+    static int n;
+    return n;
+}
+
 // expected-no-diagnostics
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */