[NFC] Fix new warnings in the headers

The newer version of clang will issue warnings in more cases,
specifically, -Winvalid-offsetof and -Wunused-but-set-variable.
This cleans up the new warnings issued from header, which shows up for
every TU that includes them. Fixing them should make the remaining
warnings easier to read.

Author: cachemeifyoucan

include/swift/Basic/HeaderFooterLayout.h

@@ -22,7 +22,10 @@ class size_without_trailing_padding {
   struct ExtraByte { char _size_without_trailing_padding_probe; };
   struct Probe: T, ExtraByte {};
 public:
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Winvalid-offsetof"
   enum { value = offsetof(Probe, _size_without_trailing_padding_probe) };
+#pragma clang diagnostic pop
 };
 
 namespace detail {

include/swift/SIL/InstWrappers.h

@@ -60,7 +60,7 @@ struct LoadOperation {
   /// TODO: Rather than use an optional here, we should include an invalid
   /// representation in LoadOwnershipQualifier.
   std::optional<LoadOwnershipQualifier> getOwnershipQualifier() const {
-    if (auto *lbi = value.dyn_cast<LoadBorrowInst *>()) {
+    if (value.dyn_cast<LoadBorrowInst *>()) {
       return std::nullopt;
     }
 

include/swift/SIL/SILInstruction.h

@@ -1195,9 +1195,12 @@ class SingleValueInstruction : public SILInstruction, public ValueBase {
 };
 
 struct SILNodeOffsetChecker {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Winvalid-offsetof"
   static_assert(offsetof(SingleValueInstruction, kind) ==
                 offsetof(NonSingleValueInstruction, kind),
                 "wrong SILNode layout in SILInstruction");
+#pragma clang diagnostic pop
 };
 
 inline SILNodePointer::SILNodePointer(const SingleValueInstruction *svi) :