Support: Use std::unique_ptr for SignpostEmitter::Impl, NFC, 3rd attempt

This reverts commit e35afbe535f96086141f57a5ce7d679429b4405f, reapplying
022ccedde8877e877b45e49641544b5e4fff0b42 and
e7ed5c920db3f537a85d962c1c918a1bb6de99fd.

- The first attempt missed defining `SignpostEmitterImpl`.
- The second attempt missed defining `llvm::SignpostEmitterImpl`.

Not sure how I failed to test both versions locally before; I thought
I'd turned the feature off via rerunning `cmake` but it must have been
stuck in place. This time I confirmed via `clang -E` that I was testing
both build configurations.

Original commit message:

    Replace some manual memory management with std::unique_ptr.

    Differential Revision: https://reviews.llvm.org/D100151

1 files changed, 5 insertions, 8 deletions

diff --git a/llvm/lib/Support/Signposts.cpp b/llvm/lib/Support/Signposts.cpp
index b277bb0..37e16a9 100644
--- a/llvm/lib/Support/Signposts.cpp
+++ b/llvm/lib/Support/Signposts.cpp
@@ -90,6 +90,9 @@ public:
   }
 };
 } // end namespace llvm
+#else
+/// Definition necessary for use of std::unique_ptr in SignpostEmitter::Impl.
+class llvm::SignpostEmitterImpl {};
 #endif // if LLVM_SUPPORT_XCODE_SIGNPOSTS
 
 #if LLVM_SUPPORT_XCODE_SIGNPOSTS
@@ -100,17 +103,11 @@ public:
 
 SignpostEmitter::SignpostEmitter() {
 #if HAVE_ANY_SIGNPOST_IMPL
-  Impl = new SignpostEmitterImpl();
-#else  // if HAVE_ANY_SIGNPOST_IMPL
-  Impl = nullptr;
+  Impl = std::make_unique<SignpostEmitterImpl>();
 #endif // if !HAVE_ANY_SIGNPOST_IMPL
 }
 
-SignpostEmitter::~SignpostEmitter() {
-#if HAVE_ANY_SIGNPOST_IMPL
-  delete Impl;
-#endif // if HAVE_ANY_SIGNPOST_IMPL
-}
+SignpostEmitter::~SignpostEmitter() = default;
 
 bool SignpostEmitter::isEnabled() const {
 #if HAVE_ANY_SIGNPOST_IMPL