[GDBJIT] Generate more complete debug information (dotnet/coreclr#7617)

[GDBJIT] Generate more complete debug information (dotnet/coreclr#7617)

* Implement support for classes
* Add support for 'this' argument type
* System.String debug info
* Functions are generated as class members in DWARF, use TypeKey as type key.
* Initial support of references
* Support 'this' artificial argument in gdbjit
* Resolve issue with recursive types in gdbjit
* DWARF classes are terminated even if they don't have children
* Fix System.Char encoding.
* Add support static fields for classes
* Add support for single dimension arrays.
* Simplify signature parsing.
* Preliminary support for multidimensional arrays.
* Add support of value types

Commit migrated from https://github.com/dotnet/coreclr/commit/3a9ee939d1043f8347ce94a49bb38d517e3bb52d