[clang] [clang][CoverageMapping] do not emit gap when either end is an `ImplicitValueInitExpr` (PR #89564)

[Zequan Wu via cfe-commits]

ZequanWu wrote:

There could be some other statements with invalid source locations (we have seen that before). I suggest validating both source locations inside `findGapAreaBetween` and returning `std::nullopt` if either of them is invalid.

https://github.com/llvm/llvm-project/pull/89564