Revert "[IRGen] The "default" LLVM code model is now Small."

This reverts commit 7526d07.

That commit was applied to master-next for changes in swift-llvm's
upstream-with-swift branch that are not included in the swift-4.1-branch.

Author: bob-wilson

lib/IRGen/IRGen.cpp

@@ -509,7 +509,7 @@ swift::createTargetMachine(IRGenOptions &Opts, ASTContext &Ctx) {
 
 
   // Create a target machine.
-  auto cmodel = CodeModel::Small;
+  auto cmodel = CodeModel::Default;
 
   // On Windows 64 bit, dlls are loaded above the max address for 32 bits.
   // This means that a default CodeModel causes generated code to segfault