[Sema] cast to CXXRecordDecl correctly when diag a default comparison method

Fixed: https://github.com/llvm/llvm-project/issues/62791
Fixed: https://github.com/llvm/llvm-project/issues/62102
in c++20, default comparison is supported. `getLexicalDeclContext` maybe cannot
get the `CXXRecord` if default comparison defined out of `CXXRecord`.
This patch want to get these information from the first function argument.

Reviewed By: #clang-language-wg, erichkeane

Differential Revision: https://reviews.llvm.org/D151365