[Support][CommandLine] Make it possible to get error messages from...

[Support][CommandLine] Make it possible to get error messages from ParseCommandLineOptions when ignoring errors.

Summary:
Previously, ParseCommandLineOptions returns false and ignores error messages
when IgnoreErrors. It would be useful to also return error messages if users
decide to check parsing result instead of having the program exit on error.

Reviewers: chandlerc, mehdi_amini, rnk

Reviewed By: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30893

llvm-svn: 297810