ClangImporter: Import (some) vector types.

When -enable-simd-import is active, if we encounter a vector type, try to load the SIMD Swift module, and if successful, map float, double, and int vectors to SIMD.{Float,Double,Int}N types if they exist.

Swift SVN r27367

lib/Frontend/CompilerInvocation.cpp

@@ -618,6 +618,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableCFunctionPointers |=
       Args.hasArg(OPT_enable_c_function_pointers);
   
+  Opts.EnableSIMDImport |=
+      Args.hasArg(OPT_enable_simd_import);
+
   Opts.EnableCharacterLiterals |= Args.hasArg(OPT_enable_character_literals);
 
   if (auto A = Args.getLastArg(OPT_enable_access_control,