[CIR] Lower __builtin_assume_dereferenceable (#197262)
`__builtin_assume_dereferenceable` was reported NYI by ClangIR. It's
heavily exercised by libcxx via `__memory/valid_range.h`, so most of
`<algorithm>` currently fails to compile under `-fclangir`.
Adds a `cir.assume_dereferenceable` op (pointer + pointer-sized
integer), emits it from CIRGenBuiltin, and lowers it to
```
call void @llvm.assume(i1 true) [ "dereferenceable"(ptr, i64) ]
```
which is what classic Clang's `Builder.CreateDereferenceableAssumption`
produces.
The size operand is widened/narrowed to `intptr_t` at the CIRGen layer
to match classic CodeGen's `IntPtrTy` conversion.
Tests cover the dynamic-size, constant-size, and narrow-integer-size
cases against CIR, CIR->LLVM, and OGCG.
