[Driver] Add an option for createInvocationFromCommandLine to recover on errors

Summary:
Previously, it would always return nullptr on any error.
This change adds a parameter, controlling whether the function should
attempt to return a non-null result even if unknown arguments (or other
errors were encountered).

The new behavior is only used in clangd.

Considered an alternative of changing the return value instead of adding
a new parameter, but that would require updating all callsites. Settled
with the parameter to minimize the code changes.

Reviewers: gribozavr

Reviewed By: gribozavr

Subscribers: nridge, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66731

llvm-svn: 370033