IRGen: Fix Clang type conversion to map Swift.Bool to _Bool instead of signed char (#2737)

CoreFoundation on Linux expects this. I'm not sure what changed
recently to expose this problem -- and I *am* sure that this
workaround is not the right way of doing it.

However, the public CI on Linux has been broken for a week now
because of this, so let's get it passing again so we can investigate
what changed in Clang/LLVM here.

Fixes <rdar://problem/26444628>.

Author: slavapestov

lib/IRGen/GenClangType.cpp

@@ -742,7 +742,9 @@ clang::CanQualType ClangTypeConverter::convert(IRGenModule &IGM, CanType type) {
       // FIXME: Handle _Bool and DarwinBoolean.
       auto &ctx = IGM.getClangASTContext();
       auto &TI = ctx.getTargetInfo();
-      if (TI.useSignedCharForObjCBool()) {
+      // FIXME: Figure out why useSignedCharForObjCBool() returns
+      // 'true' on Linux
+      if (IGM.ObjCInterop && TI.useSignedCharForObjCBool()) {
         return ctx.SignedCharTy;
       }
     }