[clang] [clang][dataflow] Clear `ExprToLoc` and `ExprToVal` at the start of a block. (PR #72985)

[via cfe-commits]

martinboehme wrote:

Update: I've run some tests on an internal codebase, and with this PR, Crubit's nullability check sees a reduction in SAT solver timeouts of over 40% and a reduction in "reached maximum iterations" errors of over 60% -- similar to the previous more complicated approach in #72850.

https://github.com/llvm/llvm-project/pull/72985