Add a SILGenBuilder constructor that takes a new insertion point.

This is the best way to inherit the provided SILGenBuilder's debug
scope and context, but insert at a different SIL location.

Author: atrick

lib/SILGen/SILGenBuilder.h

@@ -63,6 +63,14 @@ class SILGenBuilder : public SILBuilder {
                 SILBasicBlock::iterator insertInst)
       : SILGenBuilder(SGF, &*insertBB, insertInst) {}
 
+  // Create a new builder, inheriting the given builder's context and debug
+  // scope.
+  SILGenBuilder(SILGenBuilder &builder, SILBasicBlock *insertBB)
+    : SILBuilder(insertBB, builder.getCurrentDebugScope(),
+                 builder.getBuilderContext()),
+      SGF(builder.SGF)
+  {}
+
   SILGenModule &getSILGenModule() const;
   SILGenFunction &getSILGenFunction() const { return SGF; }