[Symbol] Use llvm::Expected when getting TypeSystems

Summary:
This commit achieves the following:

• Functions used to return a TypeSystem * return an llvm::Expected<TypeSystem *> now. This means that the result of a call is always checked, forcing clients to move more carefully.
• TypeSystemMap::GetTypeSystemForLanguage will either return an Error or a non-null pointer to a TypeSystem.

Reviewers: JDevlieghere, davide, compnerd

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D65122