Breakpad DWARF: Add support for DWARF 4 attribute forms.

This patch allows Breakpad's DWARF reader to at least read or skip
attributes using the new forms defined in version 4 of the DWARF
specification, instead of crashing.

Attributes encoded using DW_FORM_flag_present, DW_FORM_sec_offset, and
DW_FORM_exprloc should work fine now. However, compilation units using
DW_FORM_ref_sig8 to refer to types in .debug_types will need further work
to support. (GCC 4.6.2 does not emit .debug_types sections.)

Specifically:

- dwarf2reader::DwarfForm gets new values.
- dwarf2reader::Dwarf2Handler and dwarf2reader::DIEHandler get new handler
  methods, named ProcessAttributeSignature, for DW_FORM_ref_sig8 attributes.
- dwarf2reader::CompilationUnit reads DW_FORM_ref_sig8 attributes, and
  passes them to ProcessAttributeSignature. It also gets support for
  DW_FORM_sec_offset, DW_FORM_exprloc, and DW_FORM_flag_present, using the
  existing appropriate ProcessAttribute* methods.
- dwarf2reader::DIEDispatcher passes through ProcessAttributeSignature
  attributes to its DIEHandler.
- Unit tests are updated.

a=jimb, r=ted.mielczarek
Review URL: http://breakpad.appspot.com/343003/


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@912 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/common/dwarf/dwarf2diehandler.cc

@@ -183,4 +183,14 @@ void DIEDispatcher::ProcessAttributeString(uint64 offset,
   current.handler_->ProcessAttributeString(attr, form, data);
 }
 
+void DIEDispatcher::ProcessAttributeSignature(uint64 offset,
+                                              enum DwarfAttribute attr,
+                                              enum DwarfForm form,
+                                              uint64 signature) {
+  HandlerStack &current = die_handlers_.top();
+  // This had better be an attribute of the DIE we were meant to handle.
+  assert(offset == current.offset_);
+  current.handler_->ProcessAttributeSignature(attr, form, signature);
+}
+
 } // namespace dwarf2reader