swift234

[Driver] Add a -gdwarf-types option and set it up as an alias for -g (NFC-ish)

Background
----------

Now that Swift AST type support in LLDB has matured, we can stop emitting DWARF
type information by default to reduce compile time and ibject file size.
A future commit will change -g to emit only AST type references.

The full set of debug options will be
-gnone
-gline-tables-only
-g                 // AST types (= everything that LLDB needs)
-gdwarf-types      // AST types + DWARF types (for legacy debuggers)