c++: redundant explicit 'this' capture before C++20 [PR100493]

As described in detail in the PR, in C++20 implicitly capturing 'this'
via a '=' capture default is deprecated, and in C++17 adding an explicit
'this' capture alongside a '=' capture default is diagnosed as redundant
(and is strictly speaking ill-formed).  This means it's impossible to
write, in a forward-compatible way, a C++17 lambda that has a '=' capture
default and that also captures 'this' (implicitly or explicitly):

  [=] { this; }      // #1 deprecated in C++20, OK in C++17
		     // GCC issues a -Wdeprecated warning in C++20 mode

  [=, this] { }      // #2 ill-formed in C++17, OK in C++20
		     // GCC issues an unconditional warning in C++17 mode

This patch resolves this dilemma by downgrading the warning for #2 into
a -pedantic one.  In passing, move it into the -Wc++20-extensions class
of warnings and adjust its wording accordingly.

	PR c++/100493

gcc/cp/ChangeLog:

	* parser.c (cp_parser_lambda_introducer): In C++17, don't
	diagnose a redundant 'this' capture alongside a by-copy
	capture default unless -pedantic.  Move the diagnostic into
	-Wc++20-extensions and adjust wording accordingly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/lambda-this1.C: Adjust expected diagnostics.
	* g++.dg/cpp1z/lambda-this8.C: New test.
	* g++.dg/cpp2a/lambda-this3.C: Compile with -pedantic in C++17
	to continue to diagnose redundant 'this' captures.