[clang] Warn by default that implicit capture of 'this' is deprecated in C++20 and later.

Previously, a warning that C++20 deprecated implicit capture of 'this' for
lambda captures specified with a capture default of '=' was only issued when
'-Wdeprecated' or '-Wdeprecated-this-capture' was specified. This change
enables the warning by default (it is still only issued when compiling for
C++20 or later). This is consistent with gcc which warns by default (MSVC
requires '/Wall').

Reviewed By: erichkeane, shafik

Differential Revision: https://reviews.llvm.org/D142639