[flang] Partial implementation of Symbols and Scopes.

A Symbol consists of a common part (in class Symbol) containing name,
owner, attributes. Information for a specific kind of symbol is in a
variant containing one of the *Details classes. So the kind of symbol is
determined by the type of details class stored in the details_ variant.

For scopes there is a single Scope class with an enum indicating the
kind. So far there isn't a need for extra kind-specific details as with
Symbols but that could change. Symbols defined in a Scope are stored
there in a simple map.

resolve-names.cc is a partial implementation of a parse-tree walker that
resolves names to Symbols. Currently is only handles functions (which
introduce a new Scope) and entity-decls. The test-type executable was
reused as a driver for this to avoid the need for a new one.

Sample output is below. When each "end function" is encountered the
scope is dumped, which shows the symbols defined in it.

$ cat a.f90
pure integer(8) function foo(arg1, arg2) result(res)
  integer :: arg1
  real :: arg2
contains
  function bar(arg1)
    real :: bar
    real :: arg1
  end function
end function

$ Debug/tools/f18/test-type a.f90
Subprogram scope: 0 children
  arg1:  Entity type: REAL
  bar:  Entity type: REAL
Subprogram scope: 1 children
  arg1:  Entity type: INTEGER
  arg2:  Entity type: REAL
  bar:  Subprogram (arg1)
  foo:  Subprogram (arg1, arg2) result(res)
  res:  Entity type: INTEGER(8)

Original-commit: flang-compiler/f18@1cd2fbc04da1d6bb2ef5bc1cf07c808460ea7547
Reviewed-on: https://github.com/flang-compiler/f18/pull/30
Tree-same-pre-rewrite: false