swift234

AST: Fix mangling for nested generic types

Change the 'G' mangling to include generic parameters from
all levels of nested nominal types, and not just the innermost.

Note that the raw mangling syntax is something like this for
a nested type 'A<Int>.B<String>':

- bound_generic
  - struct 'B'
    - struct 'A'
      - module 'M'
  - args
    - Int
  - args
    - String

However, the actual mangling tree is more along the lines of:

- bound_generic_struct 'B'
  - bound_generic_struct 'A'
    - module 'M'
    - args
      - Int
  - args
    - String

This arrangement improves the quality of substitutions (we are
more likely to have a substitution for the entire unbound
generic type name 'A.B' around), and simplifies a few other
details.

Unfortunately, the remangling logic becomes slightly grotesque.

A simple SILGen test for nested generics exercises the mangling,
and ensures that Sema and SILGen do not crash with nested generics.

More detailed SILGen tests, as well as IRGen support for nested
generics is next.