Subzero: Use C++11 member initializers where practical.

Also change the pattern "foo() {}" into "foo() = default;" for ctors and dtors.

Generally avoids initializing unique_ptr<> members to nullptr in a .h file, because that requires knowing the definition of the underlying class which may not be available to all includers.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1197223002
diff --git a/src/IceDefs.h b/src/IceDefs.h
index 0e8cdfb..64e70de 100644
--- a/src/IceDefs.h
+++ b/src/IceDefs.h
@@ -201,7 +201,7 @@
   ErrorCode &operator=(const ErrorCode &) = delete;
 
 public:
-  ErrorCode() : HasError(false) {}
+  ErrorCode() = default;
   void assign(ErrorCodes Code) {
     if (!HasError) {
       HasError = true;
@@ -211,7 +211,7 @@
   void assign(int Code) { assign(static_cast<ErrorCodes>(Code)); }
 
 private:
-  bool HasError;
+  bool HasError = false;
 };
 
 // Reverse range adaptors written in terms of llvm::make_range().