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/IceIntrinsics.cpp b/src/IceIntrinsics.cpp
index 9d520b0..354120b 100644
--- a/src/IceIntrinsics.cpp
+++ b/src/IceIntrinsics.cpp
@@ -225,7 +225,7 @@
   }
 }
 
-Intrinsics::~Intrinsics() {}
+Intrinsics::~Intrinsics() = default;
 
 const Intrinsics::FullIntrinsicInfo *Intrinsics::find(const IceString &Name,
                                                       bool &Error) const {