Refactor header declaration parsing

[surovic]

Header declaration parsing was disabled for llvm-3.8 and lower. The feature relies on the <clang/Index/CodegenNameGenerator.h> header which was introduced in llvm-3.9. The code in fcd/header_decls.cpp and fcd/clang/* needs a refactor anyway; maybe it would be worth investigating if the feature can work with llvm-3.8 and lower?

[surovic]

Also good thing to note is that fcd/clang/CodegenTypes.h is a header copied from clang. Don't know why it's done this way, but I think it should be possible to do it in a cleaner way.

[surovic]

Related to #28 since both use Clang/LibTooling and stuff like clang::CompilerInstance. Objects like clang::CompilerInstance and clang::ASTContext should be made only made once per fcd run.