-Keep a reference to the ASTContext inside the TranslationUnitDecl.

-Introduce Decl::getASTContext() which returns the reference from the TranslationUnitDecl that it is contained in.

The general idea is that Decls can point to their own ASTContext so that it is no longer required to "manually" keep track and make sure that you pass the correct ASTContext to Decls' methods, e.g. methods like Decl::getAttrs should eventually not require a ASTContext parameter.

llvm-svn: 74434