Implemented initial support for "-triple" option to the clang driver. This

Implemented initial support for "-triple" option to the clang driver. This
replaces the functionality previously provided by just "-arch" (which is still
supported but has different semantics).

The new behavior is as follows:

(1) If the user does not specify -triple:

   (a) If no -arch options are specified, the target triple used is the host
   triple (in llvm/Config/config.h).

   (b) If one or more -arch's are specified (and no -triple), then there is
       one triple for each -arch, where the specified arch is substituted
       for the arch in the host triple.  Example:
          host triple = i686-apple-darwin9
          command: clang  -arch ppc -arch ppc64 ...
          triples used: ppc-apple-darwin9  ppc64-apple-darwin9

(2) The user does specify a -triple (only one allowed):

  (a) If no -arch options are specified, the triple specified by -triple
      is used.  E.g clang -triple i686-apple-darwin9

  (b) If one or more -arch options are specified, then the triple specified
      by -triple is used as the primary target, and the arch's specified
      by -arch are used to create secondary targets.  For example:

      clang -triple i686-apple-darwin9 -arch ppc -arch ppc64

      has the following targets:
         i686-apple-darwin9  (primary target)
         ppc-apple-darwin9
         ppc64-apple-darwin9

Other changes related to the changes to the driver:

- TargetInfoImpl now includes the triple string.

- TargetInfo::getTargetTriple returns the triple for its primary target.

- test case test/Parser/portability.c has been updated because "-arch linux" is
  no longer valid ("linux" is an OS, not an arch); instead we use a bogus
  architecture "bogusW16W16" where WCharWidth=16 and WCharAlign=16.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@44551 91177308-0d34-0410-b5e6-96231b3b80d8