src: improve debug info printed when LLNODE_DEBUG=true

[joyeecheung]

Part of my investigation of #143, but I think we can get this in first. This helps finding the missing symbols and unknown types so we would know what metadata needs an update .

Refs: nodejs/post-mortem#50

cc @cjihrig @bnoordhuis

BTW here is the ouput of LLNODE_CORE=core.inspect.8.9.0 LLNODE_NODE_EXE=node.linux.8.9.0 gmake _travis > /dev/null |& grep llv8 > out.txt with Node v9.2.0 on MacOS (I used prepared core file and a linux binary to speed up the scan-test), the Unknown HeapObject Type 211 is from my local changes (the Error type).

[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_off_fp_marker
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_SharedFunctionInfo__inferred_name__String
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_off_fp_marker
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_SharedFunctionInfo__inferred_name__String
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_prop_type_mask
[TEST][13193] [stderr] [llv8] Failed to load raw constant prop_type_mask
[TEST][13193] [stderr] [llv8] Unknown HeapObject Type 211 at 0x0000357d8ed67e01
[TEST][13193] [stderr] [llv8] Unknown field Type 4614
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_Context__closure_index__int
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_Context__previous_index__int
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_Context__min_context_slots__int
[TEST][13193] [stderr] [llv8] Unknown field Type 518
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_SharedFunctionInfo__inferred_name__String
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_prop_type_mask
[TEST][13193] [stderr] [llv8] Failed to load raw constant prop_type_mask
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_NameDictionaryShape__entry_size__int
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_NameDictionary__prefix_start_index__int
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_NameDictionaryShape__prefix_size__int
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_off_fp_marker
[TEST][13193] [stderr] [llv8] Failed to find symbol v8dbg_class_SharedFunctionInfo__inferred_name__String

[joyeecheung]

One error on v8.x due to #150

[joyeecheung]

Ping @bnoordhuis @cjihrig @indutny Can you review this? Thanks!

[joyeecheung]

@bnoordhuis Updated, PTAL. The tag for the constructor should be __attribute__((format(printf, 3, 4)) since there is a this

[joyeecheung]

@bnoordhuis Thanks for the review. Fixed the nits, only one known failure on the CI.