introspector: refine type extraction

[DavidKorczynski]

Example output currently:

- "name": "_ZNK6icu_7513MessageFormat6formatEiPKvPKNS_11FormattableEPKNS_13UnicodeStringEiRNS_17AppendableWrapperEPNS_13FieldPositionER10UErrorCode"
  "params":
  - "name": "this"
    "type": "int32_t"
  - "name": "msgStart"
    "type": "const void *"
  - "name": "plNumber"
    "type": "const Formattable *"
  - "name": "arguments"
    "type": "const UnicodeString *"
  - "name": "argumentNames"
    "type": "int32_t"
  - "name": "cnt"
    "type": "AppendableWrapper &"
  - "name": "appendTo"
    "type": "FieldPosition *"
  - "name": "ignore"
    "type": "UErrorCode &"
  "return_type": "void"
  "signature": "void icu_75::MessageFormat::format(int32_t, const void *, const Formattable *, const UnicodeString *, int32_t, AppendableWrapper &, FieldPosition *, UErrorCode &)"
"language": "c++"
"project": "icu"
"target_name": "converter_fuzzer"
"target_path": "/src/icu/icu4c/source/test/fuzzer/converter_fuzzer.cpp"

[DonggeLiu]

Ops, lint failure : )

[DavidKorczynski]

Ops, lint failure : )

I'll keep it a draft for just a bit, will fix the CI before making it ready :)