[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.

Reverted in fcf4d5e4 since a few dump()
functions in lldb where missed.