add a new DwarfPrinter::EmitEncodingByte method which handles

pretty printing encoding comments and eliminates redundancy on
the client side.  We now get pretty dwarf like this again:

        .byte   255                                         ## @LPStart Encoding = omit
        .byte   0                                           ## @TType Encoding = absptr
        .byte   0x28                                        ## @TType base offset
        .byte   3                                           ## Call site Encoding = udata4
        .byte   0x1a                                        ## Call site table size
...

llvm-svn: 94239