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

Patch by Jake VanAdrighem

Differential Revision: http://reviews.llvm.org/D22608

llvm-svn: 276517

clang/lib/CodeGen/BackendUtil.cpp

@@ -453,6 +453,7 @@ void EmitAssemblyHelper::CreatePasses(legacy::PassManager &MPM,
     MPM.add(createInstrProfilingLegacyPass(Options));
   }
   if (CodeGenOpts.hasProfileIRInstr()) {
+    PMBuilder.EnablePGOInstrGen = true;
     if (!CodeGenOpts.InstrProfileOutput.empty())
       PMBuilder.PGOInstrGen = CodeGenOpts.InstrProfileOutput;
     else