Change tests for ir_visibility_external, which should really test for entity_has_definition(), since the meaning of ir_visibility_external changed.

For global variables defined in the current translation unit this restores that
- they are not accessed via trampolines when compiling PIC,
- they are not skipped when emitting debug info.

This is used to implement C99 "inline"/GNU89 "extern inline" ie. an
entity which is externally defined but where we know an inlineable
definition anyway.

This commit removes the strange differentiation between
ir_visibility_external and ir_visibility_default. We now only have
ir_visibility_external for all symbols visible across compilation units.
You may or may not attach graphs/initializers to them.

(This happens when you break on a Sel creation and the firm debugger
 tries to tell you about the created node)

- We now always output dwarf3 so we can use DW_OP_call_frame_cfa and avoid
  construction location lists, but just reuse the callframe info lists.
- Backends have to emit debug info as callframe calculation changes:
  The ia32 backend has a preliminary implementation which assumes esp
  offset of frame_type_size at the beginning of a block (currently
  always true), the no-omit-fp mode assumes ebp relative addressing
  (which is correct except for the prolog/epilogue insns)

Remove the stabs debugoutput code and with it all the complexity of a
modular debug output system. The only remaining debug output module is
dwarf.

It's an internal thing and nothing from the dwarf spec

- the "create" command now catches new nodes, entities and types as
  their numbers are unique anyway

DWARF: Actually emit the list of files, not just the first filename over and over again, i.e. use index i, not 0.

this fixes a bug, because match_arguments determines the operations
mode by looking at the passed operands, so they must still have the
original mode and not the mode after a skip_downconv.

Merge a block with its only control flow predecessor,
if it contains no phi nodes and the block is the only successor of the predecessor.