[DebugInfo] Fix memory leak in DebugSSAUpdater (#159107)

Fixes an issue in commit 3946c50, PR #135349.

The DebugSSAUpdater class performs raw pointer allocations. It frees
these properly in reset(), but does not do so in its destructor - as an
immediate fix, this patch adds a destructor which frees the allocations
correctly.

I'll be merging this immediately to fix the issue, but will be open to
post-commit review and/or producing a better fix in a follow-up commit.

Author: SLTozer

llvm/include/llvm/Transforms/Utils/DebugSSAUpdater.h

@@ -235,6 +235,11 @@ class DebugSSAUpdater {
   DebugSSAUpdater(const DebugSSAUpdater &) = delete;
   DebugSSAUpdater &operator=(const DebugSSAUpdater &) = delete;
 
+  ~DebugSSAUpdater() {
+    for (auto &Block : BlockMap)
+      delete Block.second;
+  }
+
   void reset() {
     for (auto &Block : BlockMap)
       delete Block.second;