[BOLTDIFF] Add a tool to audit performance differences

Summary:
This is a simple bolt-based tool that instantiates two
RewriteInstances objects and compares them. Add a method to
RewriteInstance to enable us to compare two objects. Include a mechanism
to match functions from binary 1 to binary 2 and finally print the
largest differences in profiling data from one binary to another.

(cherry picked from FBD6517076)

bolt/CMakeLists.txt

@@ -67,6 +67,7 @@ add_llvm_tool(llvm-bolt
   BinaryFunctionProfile.cpp
   BinaryPassManager.cpp
   BinarySection.cpp
+  BoltDiff.cpp
   CacheMetrics.cpp
   DataAggregator.cpp
   DataReader.cpp
@@ -84,3 +85,4 @@ add_llvm_tool(llvm-bolt
   )
 
 add_llvm_tool_symlink(perf2bolt llvm-bolt)
+add_llvm_tool_symlink(llvm-boltdiff llvm-bolt)