c++: mangling of array new [PR119316]

c++: mangling of array new [PR119316]

Because we build an array type to represent an array new, we hit a VLA
error in compute_array_index_type for a variable length array new.  To avoid
this, let's build the MINUS_EXPR and index type directly.

I also noticed that the non-constant case in write_array_type was assuming
MINUS_EXPR without verifying it, so I added a checking_assert.

I also noticed that Clang doesn't mangle the length of an array new at all,
so I opened https://github.com/itanium-cxx-abi/cxx-abi/issues/199 to clarify
this.

	PR c++/119316

gcc/cp/ChangeLog:

	* mangle.cc (write_expression) [NEW_EXPR]: Avoid using
	compute_array_index_type.
	(write_array_type): Add checking_assert.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/mangle-new1.C: New test.