Actually run Obj-C++ tests if llvm-gcc supports.

Before there were two problems: (1) configure
turned "obj-c++" into "obj" in the langs line;
(2) the dejagnu library called it objc++ not
obj-c++.
Now the problem is that some of these tests don't
pass!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57167 91177308-0d34-0410-b5e6-96231b3b80d8

test/FrontendObjC++/dg.exp

@@ -1,5 +1,5 @@
 load_lib llvm.exp
 
-if [ llvm_gcc_supports objc++ ] then {
+if [ llvm_gcc_supports obj-c++ ] then {
     RunLLVMTests [lsort [glob -nocomplain $srcdir/$subdir/*.{mm}]]
 }