[lldb] Remove lldb's own ASTDumper

Summary:
LLDB's ASTDumper is just a clone of Clang's ASTDumper but with some scary code and
some unrelated functionality (like dumping name/attributes of types). This removes LLDB's ASTDumper
and replaces its uses with the `ClangUtils::DumpDecl` method that just calls Clang's ASTDumper
and returns the result as a string.

The few uses where we just want a textual representation of a type (which will print their name/attributes but not
dump any AST) are now also in ClangUtil under a `ToString` name until we find a better home for them.

Reviewers: labath

Reviewed By: labath

Subscribers: mgorny, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70663