Avoid ASTContext.h -> TargetInfo.h dep

This has been done before in 2008: ab138570
But these things regress easily.
Move some things out of line.

Saves 316 includes + transitive stuff:
    316 -    ../clang/include/clang/Basic/TargetOptions.h
    316 -    ../clang/include/clang/Basic/TargetInfo.h
    316 -    ../clang/include/clang/Basic/TargetCXXABI.h
    316 -    ../clang/include/clang/Basic/OpenCLOptions.h
    316 -    ../clang/include/clang/Basic/OpenCLExtensions.def
    302 -    ../llvm/include/llvm/Target/TargetOptions.h
    302 -    ../llvm/include/llvm/Support/CodeGen.h
    302 -    ../llvm/include/llvm/MC/MCTargetOptions.h
    302 -    ../llvm/include/llvm/ADT/FloatingPointMode.h
    302 -    ../clang/include/clang/Basic/XRayInstr.h
    302 -    ../clang/include/clang/Basic/DebugInfoOptions.h
    302 -    ../clang/include/clang/Basic/CodeGenOptions.h
    302 -    ../clang/include/clang/Basic/CodeGenOptions.def
    257 -    ../llvm/include/llvm/Support/Regex.h
     79 -    ../llvm/include/llvm/ADT/SmallSet.h
     68 -    MSVCSTL/include/set
     66 -    ../llvm/include/llvm/ADT/SmallPtrSet.h
     62 -    ../llvm/include/llvm/ADT/StringSwitch.h