[clang][NFC] Store a pointer to the ASTContext in ASTDumper and TextNodeDumper

In general there is no way to get to the ASTContext from most AST nodes
(Decls are one of the exception). This will be a problem when implementing
the rest of APValue::dump since we need the ASTContext to dump some kinds of
APValues.

The ASTContext* in ASTDumper and TextNodeDumper is not always
non-null. This is because we still want to be able to use the various
dump() functions in a debugger.

No functional changes intended.