Start hoisting the logic for computing the target triple into its own

function. The logic for this, and I want to emphasize that this is the
logic for computing the *target* triple, is currently scattered
throughout various different HostInfo classes ToolChain factoring
functions. Best part, it is largely *duplicated* there. The goal is to
hoist all of that up to here where we can deal with it once, and in
a consistent manner.

Unfortunately, this uncovers more fun problems: the ToolChains assume
that the *actual* target triple is the one passed into them by these
factory functions, while the *host* triple is the one in the driver.
This already was a lie, and a damn lie, when the '-target' flag was
specified. It only really worked when the difference stemmed from '-m32'
and '-m64' flags. I'll have to fix that (and remove all the FIXMEs I've
introduced here to document the problem) before I can finish hoisting
the target-calculation logic.

It's bugs all the way down today it seems...

llvm-svn: 148839