[Profile] Use a flag to enable PGO rather than the profraw filename.

[jakev]

Summary

Enable instrumentation using a flag rather than the profile filename.

This is the other half of:
https://reviews.llvm.org/D22607

[davidxl]

lgtm

cfe/trunk/lib/CodeGen/BackendUtil.cpp

void EmitAssemblyHelper::CreatePasses(legacy::PassManager &MPM,
 
   if (CodeGenOpts.hasProfileClangInstr()) {
     InstrProfOptions Options;
     Options.NoRedZone = CodeGenOpts.DisableRedZone;
     Options.InstrProfileOutput = CodeGenOpts.InstrProfileOutput;
     MPM.add(createInstrProfilingLegacyPass(Options));
   }
   if (CodeGenOpts.hasProfileIRInstr()) {
+    PMBuilder.EnablePGOInstrGen = true;
     if (!CodeGenOpts.InstrProfileOutput.empty())
       PMBuilder.PGOInstrGen = CodeGenOpts.InstrProfileOutput;
     else
       PMBuilder.PGOInstrGen = "default_%m.profraw";
   }
   if (CodeGenOpts.hasProfileIRUse())
     PMBuilder.PGOInstrUse = CodeGenOpts.ProfileInstrumentUsePath;