Previously, the presence of an explicit parameter list was detected by
querying `getNumTypeObjects()` from the `Declarator` block of the lambda
definition. This breaks for lambdas which do not have a parameter list
but _do_ have a trailing return type; that is, both of

```
[]() -> int { return 0; };
[] -> int { return 0; };
```

would return `true` when inspecting
`LambdaExpr::hasExplicitParameters()`.

Fix this by instead querying the `LParenLoc()` from the `Declarator`'s
`FunctionTypeInfo`. If `isValid() == true`, then an explicit parameter
list must be present, and if it is `false`, then it is not.

This commit also adds `hasExplicitParameters` as an attribute to a
`LambdaExpr`'s JSON AST dump. A new test (`ast-dump-lambda-json.cpp`) is
also added to validate the fix and presence of the new attribute in the
output. `ast-dump-expr-json.cpp` is also updated to validate the new
attribute.

Fixes https://github.com/llvm/llvm-project/issues/168452.