[PATCH] Driver: Replace switch cases with ifs.

[Shankar Kalpathi Easwaran]

a) would be easier to add more options, just add another case statement
b) easy to transition state from one option to another in case of positional options. 
c) sometimes specifying the same argument multiple times is invalid, so becomes easier to check too.


http://reviews.llvm.org/D7478

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