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

[Sam McCall via cfe-commits]

https://github.com/sam-mccall approved this pull request.

Sorry for missing this!

Would be nice to have a testcase but I see it's hard to get into the crashing state (maybe even not possible via clang).
Seems worthwhile to be robust to such conditions and getDefaultArgRange appears less fragile.

https://github.com/llvm/llvm-project/pull/79296