[llvm-commits] PATCH: Add support for tracking and exposing the host triple

[Chandler Carruth]

Attached is a patch to add support for tracking and exposing the host
triple. This is one of the many steps along the way to cleaning up the
Clang driver. In many places within Clang, we need to reason about the
*host*, not the target, in order to decide how to behave. For example, this
is at issue when deciding where within the host machine we should look for
the target's toolchain. While in some cases, there is a canonical location
that the target *always* uses, in other cases both can be relevant.
Consider the Debian multiarch case where we will often want to use
'/usr/lib/<host-triple>/gcc/<target-triple>/<version>/crtbegin.o'. In other
cases, we may need to check whether the host triple might contain a
toolchain capable of targeting our target. This happens frequently when
working in a bi-arch world, where the GCC installed under
'i386-pc-linux-gnu' is actually capable of targeting 'x86_64-pc-linux-gnu'
thanks to the '-m64' flag.

Some time back an effort was undertaken which simply blanket renamed 'Host'
to 'Target' in many places. This didn't really address the fundamental
problem, and in my mind made things a bit more confusing. Thus in most
places in the driver, we treat the DefaultTargetTriple as the *host*
triple, compute the HostInfo from it, and then *modify* it to produce the
target triple. Ow. Other times, we modify it first, and assume that the
post-modification triple is still a viable host triple.

All of this gets simple if we simply have access to both triples the entire
time. We can make rational decisions about when we should and should not
use the host information or the target information. Also, it will become
immediately clear from the code which is in fact in use.

Again, this is just the first step, but it touches autoconf and a bunch of
other stuff so I wanted someone else to glance at it before I checked it
in. I'll also need someone to regenerate the config.h and the configure
scripts as I don't have easy access to the proper versions. If someone can
send me those patched files i can submit as one patch to keep the build
bots happy.
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