c++: Reject __builtin_clear_padding on non-trivially-copyable types with one exceptio...

c++: Reject __builtin_clear_padding on non-trivially-copyable types with one exception [PR102586]

As mentioned by Jason in the PR, non-trivially-copyable types (or non-POD
for purposes of layout?) types can be base classes of derived classes in
which the padding in those non-trivially-copyable types can be reused for
some real data members or even the layout can change and data members can
be moved to other positions.
__builtin_clear_padding is right now used for multiple purposes,
in <atomic> where it isn't used yet but was planned as the main spot
it can be used for trivially copyable types only, ditto for std::bit_cast
where we also use it.  It is used for OpenMP long double atomics too but
long double is trivially copyable, and lastly for -ftrivial-auto-var-init=.

The following patch restricts the builtin to pointers to trivially-copyable
types, with the exception when it is called directly on an address of a
variable, in that case already the FE can verify it is the complete object
type and so it is safe to clear all the paddings in it.

2022-03-14  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102586
gcc/
	* doc/extend.texi (__builtin_clear_padding): Clearify that for C++
	argument type should be pointer to trivially-copyable type unless it
	is address of a variable or parameter.
gcc/cp/
	* call.cc (build_cxx_call): Diagnose __builtin_clear_padding where
	first argument's type is pointer to non-trivially-copyable type unless
	it is address of a variable or parameter.
gcc/testsuite/
	* g++.dg/cpp2a/builtin-clear-padding1.C: New test.