[clang] [PR49736] [C++2b] Correctly reject lambdas with requires clause and no parameter list

This fixes http://llvm.org/PR49736 caused by implementing http://wg21.link/P1102 (https://reviews.llvm.org/rG0620e6f4b76a9725dbd82454d58c5a68a7e47074), by correctly allowing requires-clause only:
1) directly after template-parameter-list
2) after lambda-specifiers iff parameter-declaration-clause is present (2nd kind of lambda-declarator)

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D99489