Rollup merge of #97840 - durin42:llvm-15-apint, r=nikic

Rollup merge of #97840 - durin42:llvm-15-apint, r=nikic

RustWrapper: adapt to APInt API changes in LLVM 15

In https://reviews.llvm.org/D125556 upstream changed sext() and zext()
to allow some no-op cases, which previously required use of the *OrSelf()
methods, which I assume is what was going on here. The *OrSelf() methods
got removed in https://reviews.llvm.org/D125559 after two weeks of
deprecation because they came with some bonus (probably-undesired)
behavior. Since the behavior of sext() and zext() changed slightly, I
kept the old *OrSelf() calls in LLVM 14 and earlier, and only use the
new version in LLVM 15.

r? `@nikic`