[Attr] Fix `-ast-print` for `asm` attribute

Without this fix, the tests introduced here produce the following
assert fail:

```
clang: /home/jdenny/llvm/clang/include/clang/Basic/AttributeCommonInfo.h:163: unsigned int clang::AttributeCommonInfo::getAttributeSpellingListIndex() const: Assertion `(isAttributeSpellingListCalculated() || AttrName) && "Spelling cannot be found"' failed.
```

The bug was introduced by D67368, which caused `AsmLabelAttr`'s
spelling index to be set to `SpellingNotCalculated`.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D70349