debug/dwarf, debug/elf: add support for reading DWARF 4 type info

In DWARF 4 the debug info for large types is put into
.debug_type sections, so that the linker can discard duplicate
info.  This change adds support for reading type units.

Another small change included here is that DWARF 3 supports
storing the byte offset of a struct field as a formData rather
than a formDwarfBlock.

R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/56300043

1 files changed, 12 insertions, 0 deletions

diff --git a/src/pkg/debug/dwarf/entry.go b/src/pkg/debug/dwarf/entry.go
index c0c2889923..934416e6c1 100644
--- a/src/pkg/debug/dwarf/entry.go
+++ b/src/pkg/debug/dwarf/entry.go
@@ -387,3 +387,15 @@ func (r *Reader) SkipChildren() {
 		}
 	}
 }
+
+// clone returns a copy of the reader.  This is used by the typeReader
+// interface.
+func (r *Reader) clone() typeReader {
+	return r.d.Reader()
+}
+
+// offset returns the current buffer offset.  This is used by the
+// typeReader interface.
+func (r *Reader) offset() Offset {
+	return r.b.off
+}