Add ability to report when a variable's value is uninitialized,

based on information provided by the compiler.  Also add new
DWARF OP, DW_OP_GNU_uninit, for this purpose.

include/elf/dwarf2.h

@@ -540,6 +540,7 @@ enum dwarf_location_atom
     DW_OP_bit_piece = 0x9d,
     /* GNU extensions.  */
     DW_OP_GNU_push_tls_address = 0xe0,
+    DW_OP_GNU_uninit     = 0xf0,
     /* HP extensions.  */
     DW_OP_HP_unknown     = 0xe0, /* Ouch, the same as GNU_push_tls_address.  */
     DW_OP_HP_is_value    = 0xe1,