debug/dwarf: add flag_present attribute encoding.

ref: http://www.dwarfstd.org/doc/DWARF4.pdf

Update #4829

R=minux.ma, iant
CC=dave, golang-dev
https://golang.org/cl/7354043

src/pkg/debug/dwarf/const.go

@@ -207,6 +207,7 @@ const (
 	formRef8        format = 0x14
 	formRefUdata    format = 0x15
 	formIndirect    format = 0x16
+	formFlagPresent format = 0x19
 )
 
 // A Tag is the classification (the type) of an Entry.

src/pkg/debug/dwarf/entry.go

@@ -185,6 +185,10 @@ func (b *buf) entry(atab abbrevTable, ubase Offset) *Entry {
 		// flag
 		case formFlag:
 			val = b.uint8() == 1
+		case formFlagPresent:
+			// The attribute is implicitly indicated as present, and no value is
+			// encoded in the debugging information entry itself.
+			val = true
 
 		// reference to other entry
 		case formRefAddr: