C: Parser: Support parsing of c99 designated initializers

In case:

    int a[6] = { [4] = 29, [2] = 15 };
    struct point { int x, y; };
    struct point p = { .y = 3, .x = 2 };

Grammar change when c99 language feature is enabled:
old grammar:

    braced-init-list :: '{' initializer-list '}'

new grammar:

    braced-init-list :: '{' designated-initializer-list '}'
    designated-initializer-list :: designated-initializer (',' designated-initializer )*
    designated-initializer :: designator* initializer-clause
    designator :: '.' identifier
                | '[' constant-expression ']'

Task-number: QTCREATORBUG-1902
Change-Id: Ib99d6f553f8d0f50ba3eff86f3a2e86d73372426
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/libs/3rdparty/cplusplus/Parser.h

@@ -246,6 +246,11 @@ public:
 
     bool lookAtObjCSelector() const;
 
+    // c99
+    bool parseDesignatedInitializerList(ExpressionListAST *&node);
+    bool parseDesignatedInitializer(ExpressionAST *&node);
+    bool parseDesignator(DesignatorAST *&node);
+
     bool skipUntil(int token);
     void skipUntilDeclaration();
     bool skipUntilStatement();