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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189416
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