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>

tests/auto/cplusplus/cplusplus.qbs

@@ -4,6 +4,7 @@ Project {
     name: "CPlusPlus autotests"
     references: [
         "ast/ast.qbs",
+        "c99/c99.qbs",
         "checksymbols/checksymbols.qbs",
         "codeformatter/codeformatter.qbs",
         "cxx11/cxx11.qbs",