swift234

Debug Info! Add basic support for line tables, compilation units, files,
and lexical scopes, which can be enabled through the new -g option.
When -g is enabled, line tables and scopes compile all the way
down to DWARF.

Changes to SIL:
- In addition to a SILLocation, every instruction now also has a pointer
  to a SILDebugScope (its containing lexical scope).
- Added LexicalScope, which is to be used for all Scopes we want to show
  up in the debug info.

Swift SVN r5772