loplugin:stringconstant: Also handle u8"..." strings

(just in case)

Change-Id: Id8ce7436bacba874d9bc04915e42622e3cfca67a

compilerplugins/clang/stringconstant.cxx

@@ -1163,7 +1163,7 @@ bool StringConstant::isStringConstant(
     }
     clang::StringLiteral const * lit = dyn_cast<clang::StringLiteral>(expr);
     if (lit != nullptr) {
-        if (!lit->isAscii()) {
+        if (!(lit->isAscii() || lit->isUTF8())) {
             return false;
         }
         unsigned n = lit->getLength();

compilerplugins/clang/test/stringconstant.cxx

@@ -66,6 +66,8 @@ int main() {
     (void) OUString("x\xA0x", 3, RTL_TEXTENCODING_ISO_8859_1);
 
     (void) OUString("xxx", 2, RTL_TEXTENCODING_ASCII_US); // expected-error {{suspicious 'rtl::OUString' constructor with literal of length 3 and non-matching length argument 2 [loplugin:stringconstant]}}
+
+    (void) OUString(u8"xxx", 3, RTL_TEXTENCODING_ASCII_US); // expected-error {{simplify construction of 'OUString' with string constant argument [loplugin:stringconstant]}}
 }