Add findFunctionInfoList() accessor to FunctionInfoIndex.

[mehdi_amini]

Summary

This allows to query for a function in the map without creating an
entry, allowing to use a const FunctionInfoIndex.

[tejohnson]

LGTM, thanks

[mehdi_amini]

r253953

include/llvm/IR/FunctionInfo.h

public:
   funcinfo_iterator end() { return FunctionMap.end(); }
   const_funcinfo_iterator end() const { return FunctionMap.end(); }
 
   /// Get the list of function info objects for a given function.
   const FunctionInfoList &getFunctionInfoList(StringRef FuncName) {
     return FunctionMap[FuncName];
   }
 
+  /// Get the list of function info objects for a given function.
+  const const_funcinfo_iterator findFunctionInfoList(StringRef FuncName) const {
+    return FunctionMap.find(FuncName);
+  }
+
   /// Add a function info for a function of the given name.
   void addFunctionInfo(StringRef FuncName, std::unique_ptr<FunctionInfo> Info) {
     // Update the HasExportedFunctions flag, but only if we had a function
     // summary (i.e. we aren't parsing them lazily or have a bitcode file
     // without a function summary section).
     if (ExportingModule && Info->functionSummary()) {
       if (ExportingModule->getModuleIdentifier() ==
           Info->functionSummary()->modulePath())