[flang] Address review comments.

[flang] Address review comments.

Use "namespace Fortran::semantics".

Add helper MakeSymbol() functions to ResolveNameVisitor to make a symbol
in the current scope.

Start work on subroutines, similar to what's been done for functions.

Original-commit: flang-compiler/f18@afe84af1c7a16f7d5de8897d0c3453f580ac7e07
Reviewed-on: https://github.com/flang-compiler/f18/pull/30