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/IceCompiler.h b/src/IceCompiler.h
index 87915a6..460c327 100644
--- a/src/IceCompiler.h
+++ b/src/IceCompiler.h
@@ -30,7 +30,7 @@
   Compiler &operator=(const Compiler &) = delete;
 
 public:
-  Compiler() {}
+  Compiler() = default;
 
   // Run the compiler with the given GlobalContext for compilation
   // state.  Upon error, the Context's error status will be set.