[lldb] Avoid unhandled Error in TypeSystemMap::GetTypeSystemForLanguage

When assertions are turned off, the `llvm::Error` value created at the
start of this function is overwritten using the move-assignment
operator, but the success value is never checked. Whenever a TypeSystem
cannot be found or created, this can lead to lldb core dumping with:

    Program aborted due to an unhandled Error:
    Error value was Success. (Note: Success values must still be checked prior to being destroyed).

Fix this by not creating a `llvm::Error` value in advance, and directly
returning the result of `llvm::make_error` instead, whenever an error is
encountered.

See also: <https://bugs.freebsd.org/253881> and
<https://bugs.freebsd.org/257829>.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D108088

(cherry picked from commit 5033f079)