Initial implementation of Windows x86 target; at the moment, the only

Initial implementation of Windows x86 target; at the moment, the only
difference from generic x86 is the defines.  The rest is non-trivial to
implement.

I'm not planning on adding any more targets myself; if there are any
targets anyone is currently using that are missing, feel free to add
them, or ask me to add them.

This concludes the work I'm planning for the TargetInfo
implementations at the moment; all the other issues with TargetInfo require
some API changes, and I haven't really thought it through.  Some of the
remaining issues: allowing targets to define size_t and wchar_t properly,
adding some sort of __builtin_type_info intrinsic so we can finish clang's
limits.h and float.h and get rid of a massive number of macro
definitions, allowing target-specific command-line options, allowing
target-specific defaults for certain command-line options like
-fms-extensions, exposing vector alignment outside of the description
string, exposing endianness outside of the description string, allowing
targets to expose special bit-field layout requirements, exposing some
sort of custom hook for call generation in CodeGen, and adding CPU
selection to control defines like __SSE__.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@55098 91177308-0d34-0410-b5e6-96231b3b80d8