Support debugging programs written in C++

[osandov]

drgn can currently attach to C++ programs and mostly work. However, C++ obviously has many features that C doesn't, and many of drgn's APIs need updates to represent these, including but not limited to the following. We'll add more as we encounter them.

• Add cpp language stub #50
• Add support for parsing and reading C++ type templates #60
• libdrgn: dwarf_info: Support DW_TAG_GNU_template_parameter_pack #222
• libdrgn: Add support for storing and returning namespace tags #67
• libdrgn: Follow typedefs in enum backing type lookup #128
• Enable prog.type to work with classes #158
• Add support for UTF character base types #187
• Correctly interpret template parameters in the C++ lexer #223
• Add support for looking up types in namespaces #225
• Add support for reading methods via "members" array #72
• libdrgn: Ensure booleans are treated as primitives in C++ #227
• Remove template arguments from drgn.Type.tag #80
• Support C++ references #81
• Parse and format default function arguments #82
• Add accessibility (public, private, protected) to drgn.TypeMember #83
• Support C++ inheritance #84
• Support static members #85
• Support C++ friendship #86
• Record C++ virtuality of types/methods
• Support looking up types in C++ without the struct/class/union/enum keyword #348
• Support C++ enum class
• Support C++ nested classes #349
• Support C++ this
• Support C++ function/method overloads
• Record C++ default member initializers

#123 will probably be required to nicely represent a lot of these features.