Move the Linux header searching from the Frontend to the Driver. This is

the first (and diff-noisiest) step to making Linux header searching
tremendously more principled and less brittle. Note that this step
should have essentially no functional impact. We still search the exact
same set of paths in the exact same order. The only change here is where
the code implementing such a search lives.

This has one obvious negative impact -- we now pass a ludicrous number
of flags to the CC1 layer. That should go away as I re-base this logic
on the logic to detect a GCC installation. I want to do this in two
phases so the bots can tell me if this step alone breaks something, and
so that the diffs of the refactoring make more sense.

llvm-svn: 143822