[clang-format] Exclude kw_decltype in RemoveParentheses

From https://en.cppreference.com/w/cpp/language/decltype:
Note that if the name of an object is parenthesized, it is treated as an
ordinary lvalue expression, thus decltype(x) and decltype((x)) are often
different types.

Fixes #64786.

Differential Revision: https://reviews.llvm.org/D158155