Handle global entities in AST generation

[surovic]

The AST generation algorithm does not consider most global entities in the input bitcode. Implementing handling of global entities, such as aggregate type definitions, global variables, function declarations, etc., would improve the presentation of bitcode lifted from binaries and extracted from C code, and enable proper presentation of bitcode extracted from C++ code. This would also put fcd+remill closer to outputting complete valid C code.

[surovic]

Update: Progress being made in dev-clang-ast, with initial support in 086a51b.