[flang] Address review comments.

[flang] Address review comments.

Replace operator=(Symbol) and operator=(DeclTypeSpec) with set_symbol()
and set_type() to make it clearer. Eliminate the move constructor as it
is not needed. Make all but the accessor functions out-of-line.

Change HasExplicitType() to NeedsExplicitType(), reversing its sense.
HasExplicitType() returned true for symbols that didn't have or need an
explicit type, which was confusing.

Original-commit: flang-compiler/f18@ada13ac6a32145fe260d78cb0e736998e5eabd15
Reviewed-on: https://github.com/flang-compiler/f18/pull/97