dwarf: add marker for embedded fields in dwarf
    
    Currently, the following two codes generate the identical dwarf info
    for type Foo.
    
    prog 1)
    type Foo struct {
       Bar
    }
    
    prog 2)
    type Foo struct {
       Bar Bar
    }
    
    This change adds a go-specific attribute DW_AT_go_embedded_field
    to annotate each member entry. Its absence or false value indicates
    the corresponding member is not an embedded field.
    
    Update #20037
    
    Change-Id: Ibcbd2714f3e4d97c7b523d7398f29ab2301cc897
    Reviewed-on: https://go-review.googlesource.com/41873
    Reviewed-by: David Chase <drchase@google.com>