ArgList: cache index ranges containing arguments with each ID

Improve performance of argument list parsing with large numbers of IDs and
large numbers of arguments, by tracking a conservative range of indexes within
the argument list that might contain an argument with each ID. In the worst
case (when the first and last argument with a given ID are at the opposite ends
of the argument list), this still results in a linear-time walk of the list,
but it helps substantially in the common case where each ID occurs only once,
or a few times close together in the list.

This gives a ~10x speedup to clang's `test/Driver/response-file.c`, which
constructs a very large set of command line arguments and feeds them to the
clang driver.

Differential Revision: https://reviews.llvm.org/D30130

llvm-svn: 300135

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