[CastOptimizer] Set the correct debug scope for the SILBuilder.

<rdar://problem/48887384>

Author: Davide Italiano

lib/SILOptimizer/Utils/CastOptimizer.cpp

@@ -1532,7 +1532,7 @@ static bool optimizeStaticallyKnownProtocolConformance(
     if (!Conformance)
       return false;
 
-    SILBuilder B(Inst);
+    SILBuilderWithScope B(Inst);
     SmallVector<ProtocolConformanceRef, 1> NewConformances;
     NewConformances.push_back(Conformance.getValue());
     ArrayRef<ProtocolConformanceRef> Conformances =

test/SILOptimizer/castoptimizer-wrongscope.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -emit-sil -o /dev/null \
+// RUN:   %s -Xllvm -sil-print-debuginfo -Onone -sil-verify-all \
+// RUN:   -Xllvm -sil-print-after=diagnostic-constant-propagation \
+// RUN:   2>&1 | %FileCheck %s
+
+// CHECK: alloc_stack $R, loc {{.*}}, scope 2
+// CHECK-NEXT: init_existential_addr {{.*}} : $*R, $Float, loc {{.*}}, scope 2
+// CHECK-NEXT: copy_addr [take] %8 to [initialization] %66 : $*Float, loc {{.*}}, scope 2
+
+protocol R {}
+extension Float: R {}
+print(1.0 as Float? as! R)