Fix clangsa on LLVM 10 (#37828)
* [GCChecker] Fix assertion
We were associating a symbol with a `void` location. This is illegal,
but happened to let us keep track of arraylist entries. Stop that
abuse by manually modeling indexing into arraylist and fix the
GCChecker assertion.
* Backport LLVM 10 clangsa patch from Yggdrasil
