[flang] Save CallStmt::typedCall (flang-compiler/f18#879)

When `ExpressionAnalyzer::AnalyzeCall` processed a subroutine it was
always returning std::nullopt. Change it to return a `ProcedureRef`
wrapped in an `Expr` so that it can be saved in `CallStmt::typedCall`.

Original-commit: flang-compiler/f18@2cc226f66ec51b38056c779494a7b6104557f9a8
Reviewed-on: https://github.com/flang-compiler/f18/pull/879