[clang] Improve HeuristicResolver further so it can replace most of getApproximateType() in SemaCodeComplete (PR #156282)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> hi! we're seeing much higher (>100x) indexing times on certain code patterns after this change.

Thanks for the reproducer! I will investigate.

https://github.com/llvm/llvm-project/pull/156282