PMBuilder: Expose an option to disable tail calls

Adds API to allow frontends to disable tail calls in PassManagerBuilder.

<rdar://problem/16050591>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206542 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/IPO/PassManagerBuilder.h

@@ -106,6 +106,7 @@ public:
   /// added to the per-module passes.
   Pass *Inliner;
 
+  bool DisableTailCalls;
   bool DisableUnitAtATime;
   bool DisableUnrollLoops;
   bool BBVectorize;