annotate private class members as needed (#50)

## Purpose
Updates IDS to add the export annotation to private class members that
require export. Previously, only public and protected member were
exported.

## Overview
This patch makes the following changes:
1. Implements `VisitCallExpr` to visit method calls identified in the
AST. Common code for exporting method declarations was refactored from
`VisitFunctionDecl` into private method `export_function_if_needed`,
which is now also called by `VisitCallExpr` for private methods
2. Implements `VisitDeclRefExpr` to visit static fields references
identified in the AST. Common code for exporting variable declarations
was refactored from `VisitVarDecl` into `export_variable_if_needed`,
which is now also called by `VisitDeclRefExpr` for private static
fields.
3. Tracks every method, variable, and record declaration that needs an
export annotation added in a simple `DeclSet` class. An instance of this
class replaces previous `in_exported_record_ ` member bool that was used
to track if the "current" record was being exported. This change
simplifies things and ensures that we don't mistakenly export the same
symbol twice now that there are multiple codepaths.
4. Implements a new `is_in_system_header` method to ensure we skip
processing any declarations in system headers (this was missing from
`VisitVarDecl`).
5. Adds a bunch of new test cases that fail without the other changes.

## Background
Private methods in a class must be exported when they are referenced by
inline function definitions. For example:
``` C++
class Example {
public:
  inline int publicMethod() {
    return privateMethod();
  }

private:
  int privateMethod();
};
```
Similarly, private static fields must in a class must also be exported
when referenced by inline function definitions. For example:
``` C++
class Example {
public:
  inline int publicMethod() {
    return privateStaticFiedl;
  }

private:
  static int privateStaticField;
};
```

## Validation
1. Ran existing and new tests on Linux and Windows.
2. Ran on LLVM Support library headers and compared results with
previous results & manual fixups. Results matched what I had done
manually before (+ a few annotations I missed).

---------

Co-authored-by: Saleem Abdulrasool <[email protected]>

Author: andrurogerz