[flang] Track current statement location in SemanticsVisitor

Change `SemanticsVisitor` to track the location of the current
statement, if any, so that it's available through
`SemanticsContext::location()`

Add overloading of `SemanticsContext::Say()` that reports the
message at the location of the current statement if a `CharBlock`
is not provided.

Original-commit: flang-compiler/f18@5185c721ee9e2da8adcc4cad5a74acc0bf5c8d2d
Reviewed-on: https://github.com/flang-compiler/f18/pull/357