ASTContext: Declare builtin types implicitly

__builtin_va_list and friends have been showing up where they shouldn't for way
to long, making unwanted appearences in -ast-print, tooling and source level
visitors and even the hello world tutorial on the clang website.

This commit factors down the implicit typedef and record creation facilities to
ensure they're marked implicit.

Also fixes a unit test that was testing incorrect behaviour, and removes old
hacks in the DeclPrinter that tried to skip implicit declarations manually.