[flang] Normalize namespace definitions.

Replace `namespace Fortran { namespace parser { ... } }` with
`namespace Fortran::parser { ... }` and similarly for semantics.

Original-commit: flang-compiler/f18@838c9539b2497f8dd1e33bd23623a5373a694e7b
Reviewed-on: https://github.com/flang-compiler/f18/pull/77
Tree-same-pre-rewrite: false