[clang] Use getDefaultArgRange instead of getDefaultArg to retrieve the (#79296)

source location in` AliasTemplateDeductionGuideTransform`.

I don't have a reproducible testcase, but this should be a safe and
non-functional change. We have checked the `hasDefaultArg` before
calling `getDefaultArg()`, but `hasDefaultArg` allows
unparsed/uninstantiated default arg which is prohibited in
`getDefaultArg()`.

Since we're only interested in the source location, we switch to use
`getDefaultArgRange()` API.