[Basic] Drop const from a return type (NFC) (llvm#146382)

We don't need const on a return type.

Author: kazutakahirata

clang/include/clang/Basic/TargetInfo.h

@@ -1851,7 +1851,7 @@ class TargetInfo : public TransferrableTargetInfo,
 
   /// Returns the version of the darwin target variant SDK which was used during
   /// the compilation if one was specified, or an empty version otherwise.
-  const std::optional<VersionTuple> getDarwinTargetVariantSDKVersion() const {
+  std::optional<VersionTuple> getDarwinTargetVariantSDKVersion() const {
     return !getTargetOpts().DarwinTargetVariantSDKVersion.empty()
                ? getTargetOpts().DarwinTargetVariantSDKVersion
                : std::optional<VersionTuple>();