[Clang] Use of decltype(capture) in parameter-declaration-clause

Partially implement the proposed resolution to CWG2569.

D119136 broke some libstdc++ code, as P2036R3, implemented as a DR to
C++11 made ill-formed some previously valid and innocuous code.

We resolve this issue to allow decltype(x) - but not decltype((x)
to appear in the parameter list of a lambda that capture x by copy.

Unlike CWG2569, we do not extend that special treatment to
sizeof/noexcept yet, as the resolution has not been approved yet
and keeping the review small allows a quicker fix of impacted code.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D123909