[Driver] Use shared singleton instance of DriverOptTable

Summary:
This significantly reduces the time required to run clangd tests, by
~10%.

Should also have an effect on other tests that run command-line parsing
multiple times inside a single invocation.

Reviewers: gribozavr, sammccall

Reviewed By: sammccall

Subscribers: kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67163

llvm-svn: 370908