[CIR] Upstream EHScopeStack memory allocator (#152215)

When the cleanup handling code was initially upstreamed, a SmallVector
was used to simplify the handling of the stack of cleanup objects.
However, that mechanism won't scale well enough for the rate at which
cleanup handlers are going to be pushed and popped while compiling a
large program. This change introduces the custom memory allocator which
is used in classic codegen and the CIR incubator.

Thiis does not otherwise change the cleanup handling implementation and
many parts of the infrastructure are still missing.

This is not intended to have any observable effect on the generated CIR,
but it does change the internal implementation significantly, so it's
not exactly an NFC change. The functionality is covered by existing
tests.

0 files changed, 0 insertions, 0 deletions