[clang] [clang][FlowSensitive] Do a quick check and bail early for massive CFGs (PR #186808)

[Jan Voung via cfe-commits]

jvoung wrote:

> > Those examples also have lots of `Locs`, which we track in MapVectors for each BB. Since the maps do not share memory across BBs,
> 
> Aside: should we be using a different datastructure for the Locs?

I thought about that as an alternative -- say a Patricia tree... but it's not clear there will be enough sharing to enough of a difference, and I'm not sure if the tree structures will have worse cache locality and end up making the common case slower.

https://github.com/llvm/llvm-project/pull/186808