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/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 30920a4..6569626 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -2173,7 +2173,7 @@
     assert(Value && "missing value for complete object");
   }
 
-  LLVM_EXPLICIT operator bool() const { return Value; }
+  explicit operator bool() const { return Value; }
 };
 
 /// Find the designated sub-object of an rvalue.