Option parsing: support case-insensitive option matching.

Link.exe's command line options are case-insensitive. This patch
adds a new attribute to OptTable to let the option parser to compare
options, ignoring case.

Command lines are generally case-insensitive on Windows. CL.exe is an
exception. So this new attribute should be useful for other commands
running on Windows.

Differential Revision: http://llvm-reviews.chandlerc.com/D1485