gimple-fold: Fix up __builtin_clear_padding on classes with virtual inheritence ...

gimple-fold: Fix up __builtin_clear_padding on classes with virtual inheritence [PR101586]

For the following testcase, B is 16-byte type, containing 8-byte
virtual pointer and 1-byte A member, and C contains two FIELD_DECLs,
one with B type and size of just 8-byte and then a field with type
A and 1-byte size.
The __builtin_clear_padding code was upset about the B typed FIELD_DECL
containing FIELD_DECLs beyond the field size and triggered
assertion failure.
This patch makes it ignore all FIELD_DECLs that are (fully) beyond the sz
passed from the caller (except for the flexible array member
diagnostics that is kept).

2021-07-27  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101586
	* gimple-fold.c (clear_padding_type): Ignore FIELD_DECLs with byte
	positions above or equal to sz except for diagnostics of flexible
	array members.

	* g++.dg/torture/builtin-clear-padding-4.C: New test.