builtin macros

This patch implements the __is_target_arch, __is_target_vendor, __is_target_os,
and __is_target_environment Clang preprocessor extensions that were proposed by
@compnerd in Bob's cfe-dev post:
http://lists.llvm.org/pipermail/cfe-dev/2017-November/056166.html.

These macros can be used to examine the components of the target triple at
compile time. A has_builtin(is_target_???) preprocessor check can be used to
check for their availability.

__is_target_arch allows you to check if an arch is specified without worring
about a specific subarch, e.g.

__is_target_arch(arm) returns 1 for the target arch "armv7"
__is_target_arch(armv7) returns 1 for the target arch "armv7"
__is_target_arch(armv6) returns 0 for the target arch "armv7"

__is_target_vendor and __is_target_environment match the specific vendor
or environment. __is_target_os matches the specific OS, but
__is_target_os(darwin) will match any Darwin-based OS. "Unknown" can be used
to test if the triple's component is specified.

rdar://35753116

Differential Revision: https://reviews.llvm.org/D41087


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320734 91177308-0d34-0410-b5e6-96231b3b80d8