Debug info: Properly handle the function prologue in the line table.

Introduces a new flag in SILLocation: InPrologue to mark instructions
that setup the stack and allocate storage for local variables/arguments.

Fixes rdar://problem/15290023: Breakpoint set on prologue - crash ensues.

Swift SVN r9686

lib/IRGen/IRGenFunction.cpp

@@ -35,7 +35,8 @@ IRGenFunction::IRGenFunction(IRGenModule &IGM,
                              Optional<SILLocation> DbgLoc)
   : IGM(IGM), Builder(IGM.getLLVMContext()),
     CurFn(Fn), CurExplosionLevel(explosionLevel),
-    ContextPtr(nullptr) {
+    ContextPtr(nullptr),
+    DbgScope(DbgScope) {
   // Make sure the instructions in this function are attached its debug scope.
   if (IGM.DebugInfo) {
     // Functions, especially artificial thunks and closures, are often
@@ -43,7 +44,6 @@ IRGenFunction::IRGenFunction(IRGenModule &IGM,
     // function. Be nice and preserve the current debug location until
     // after we're done with this function.
     IGM.DebugInfo->pushLoc();
-    IGM.DebugInfo->setCurrentLoc(Builder, DbgScope, DbgLoc);
   }
 
   emitPrologue();