fold-const: Fix up -fsanitize=null in C++ [PR105729]

fold-const: Fix up -fsanitize=null in C++ [PR105729]

The following testcase triggers a false positive UBSan binding a reference
to null diagnostics.
In the FE we instrument conversions from pointer to reference type
to diagnose at runtime if the operand of such a conversion is 0.
The problem is that a GENERIC folding folds
((const struct Bar *) ((const struct Foo *) this)->data) + (sizetype) range_check (x)
conversion to const struct Bar & by converting to that the first
operand of the POINTER_PLUS_EXPR.  But that changes when the -fsanitize=null
binding to reference runtime check occurs.  Without the optimization,
it is invoked on the result of the POINTER_PLUS_EXPR, and as range_check
call throws, that means it never triggers in the testcase.
With the optimization, it checks whether this->data is NULL and it is.

The following patch avoids that optimization during GENERIC folding when
-fsanitize=null is enabled and it is a cast from non-REFERENCE_TYPE to
REFERENCE_TYPE.

2022-05-27  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/105729
	* fold-const.cc (fold_unary_loc): Don't optimize (X &) ((Y *) z + w)
	to (X &) z + w if -fsanitize=null during GENERIC folding.

	* g++.dg/ubsan/pr105729.C: New test.