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/IceFixups.h b/src/IceFixups.h
index 3b2b524..9ec72c3 100644
--- a/src/IceFixups.h
+++ b/src/IceFixups.h
@@ -28,7 +28,7 @@
   AssemblerFixup &operator=(const AssemblerFixup &) = delete;
 
 public:
-  AssemblerFixup() : position_(0), kind_(0), value_(nullptr) {}
+  AssemblerFixup() = default;
   AssemblerFixup(const AssemblerFixup &) = default;
   intptr_t position() const { return position_; }
   void set_position(intptr_t Position) { position_ = Position; }
@@ -47,9 +47,9 @@
   void emit(GlobalContext *Ctx, RelocOffsetT BaseOffset) const;
 
 private:
-  intptr_t position_;
-  FixupKind kind_;
-  const Constant *value_;
+  intptr_t position_ = 0;
+  FixupKind kind_ = 0;
+  const Constant *value_ = nullptr;
 };
 
 typedef std::vector<AssemblerFixup> FixupList;