Fix compilation on MSVC. Rename "_BB" to "Block"

Thanks zygoloid.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151481 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/llvm-stress/llvm-stress.cpp b/tools/llvm-stress/llvm-stress.cpp
index e99d54b..db35688 100644
--- a/tools/llvm-stress/llvm-stress.cpp
+++ b/tools/llvm-stress/llvm-stress.cpp
@@ -93,8 +93,8 @@
 
 public:
   /// C'tor
-  Modifier(BasicBlock *_BB, PieceTable *PT, Random *R):
-    BB(_BB),PT(PT),Ran(R),Context(BB->getContext()) {};
+  Modifier(BasicBlock *Block, PieceTable *PT, Random *R):
+    BB(Block),PT(PT),Ran(R),Context(BB->getContext()) {};
   /// Add a new instruction.
   virtual void Act() = 0;
   /// Add N new instructions,