Removing LLVM_EXPLICIT, as MSVC 2012 was the last reason for requiring the macro. NFC; Clang edition.

llvm-svn: 229336
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index ea8166b..dc29858 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -2280,7 +2280,7 @@
       return ConstantEmission(C, false);
     }
 
-    LLVM_EXPLICIT operator bool() const {
+    explicit operator bool() const {
       return ValueAndIsReference.getOpaqueValue() != nullptr;
     }