debug/dwarf: add support for line tables

This implements a LineReader for line tables that parallels the
existing Reader for debug entries.

This code is partly based on the debug subrepo's fork of dwarf, but it
is a more complete (and, I believe, correct) implementation of the
spec and exposes a more general API.  While the debug subrepo's
implementation exposed only a PC-to-line function, this version
exposes the line table rows to the caller.  This way the caller can
make its own trade-offs when implementing PC-to-line (or line-to-PC),
such as whether or not to build an index for fast lookup.

Change-Id: Ie157bc817f55e940b6f2e1ae010c5a4e1f29c5c8
Reviewed-on: https://go-review.googlesource.com/6734Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/debug/dwarf/const.go

@@ -452,3 +452,31 @@ const (
 	encUnsignedChar   = 0x08
 	encImaginaryFloat = 0x09
 )
+
+// Statement program standard opcode encodings.
+const (
+	lnsCopy           = 1
+	lnsAdvancePC      = 2
+	lnsAdvanceLine    = 3
+	lnsSetFile        = 4
+	lnsSetColumn      = 5
+	lnsNegateStmt     = 6
+	lnsSetBasicBlock  = 7
+	lnsConstAddPC     = 8
+	lnsFixedAdvancePC = 9
+
+	// DWARF 3
+	lnsSetPrologueEnd   = 10
+	lnsSetEpilogueBegin = 11
+	lnsSetISA           = 12
+)
+
+// Statement program extended opcode encodings.
+const (
+	lneEndSequence = 1
+	lneSetAddress  = 2
+	lneDefineFile  = 3
+
+	// DWARF 4
+	lneSetDiscriminator = 4
+)