[lldb] Provide a getter for m_materializer_up in LLVMUserExpression instead of relyin...

[lldb] Provide a getter for m_materializer_up in LLVMUserExpression instead of relying on it being accessible.

Summary:
Motivated by Swift using the materializer in a few places which requires us to add this getter ourselves.
We also need a setter, but let's keep this minimal to unblock the downstream reverts in Swift.

Reviewers: davide

Reviewed By: davide

Subscribers: abidh, JDevlieghere, lldb-commits

Tags: #upstreaming_lldb_s_downstream_patches, #lldb

Differential Revision: https://reviews.llvm.org/D69714