Make StringMap's copy ctor non-explicit.

Without this gcc doesn't allow us to put a StringMap into a
std::map. Works with clang though.

llvm-svn: 154737

llvm/include/llvm/ADT/StringMap.h

@@ -239,7 +239,7 @@ public:
   explicit StringMap(AllocatorTy A)
     : StringMapImpl(static_cast<unsigned>(sizeof(MapEntryTy))), Allocator(A) {}
 
-  explicit StringMap(const StringMap &RHS)
+  StringMap(const StringMap &RHS)
     : StringMapImpl(static_cast<unsigned>(sizeof(MapEntryTy))) {
     assert(RHS.empty() &&
            "Copy ctor from non-empty stringmap not implemented yet!");