[Clang] Fix parsing of `(auto(x))`.

Allow auto(x) to appear in a parenthesis
expression.

The pattern (auto( can appear as part of a declarator,
so the parser is modified to avoid the ambiguity,
in a way consistent with the proposed resolution to CWG1223.

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D149276