Fix assorted C++ nits caught by stricter warnings from clang-5.0

Summary: Offload the gcc-compatible fixes from the diff that cuts-over to building fb4a with clang.

Differential Revision: D6525454

fbshipit-source-id: 02ce4d8f84ebbce6a4605f0b64b822e11defe20a

Author: gkmhub

cpp/profiler/BaseTracer.h

@@ -27,17 +27,17 @@ class BaseTracer {
     ucontext_t* ucontext,
     int64_t* frames,
     uint8_t& depth,
-    uint8_t max_depth);
+    uint8_t max_depth) = 0;
 
   virtual void flushStack(
     int64_t* frames,
     uint8_t depth,
     int tid,
-    int64_t time_);
+    int64_t time_) = 0;
 
-  virtual void startTracing();
+  virtual void startTracing() = 0;
 
-  virtual void stopTracing();
+  virtual void stopTracing() = 0;
 };
 
 } // namespace profiler