[Options] Add prefixes to options.

Each option has a set of prefixes. When matching an argument such as
-funroll-loops. First the leading - is removed as it is a prefix. Then
a lower_bound search for "funroll-loops" is done against the option table by
option name. From there each option prefix + option name combination is tested
against the argument.

This allows us to support Microsoft style options where both / and - are valid
prefixes. It also simplifies the cases we already have where options come in
both - and -- forms. Almost every option for gnu-ld happens to have this form.

llvm-svn: 166444

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