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
llvm-svn: 367360