Use 'cast' instead of 'get' to fix a compiler warning.

Fixes:

```
ClangSharp/ClangSharp/sources/libClangSharp/ClangSharp.cpp:166:48: warning: 'get' is deprecated: Use cast instead [-Wdeprecated-declarations]
  166 |                     return MakeCXCursor(object.get<BlockDecl*>(), getCursorTU(C));
      |                                                ^
ClangSharp/artifacts/llvm/install/include/llvm/ADT/PointerUnion.h:161:12: note: 'get' has been explicitly marked deprecated here
  161 |   inline T get() const {
      |            ^
ClangSharp/ClangSharp/sources/libClangSharp/ClangSharp.cpp:166:48: warning: 'get<clang::BlockDecl *>' is deprecated: Use cast instead [-Wdeprecated-declarations]
  166 |                     return MakeCXCursor(object.get<BlockDecl*>(), getCursorTU(C));
      |                                                ^
ClangSharp/artifacts/llvm/install/include/llvm/ADT/PointerUnion.h:160:5: note: 'get<clang::BlockDecl *>' has been explicitly marked deprecated here
  160 |   [[deprecated("Use cast instead")]]
      |     ^
ClangSharp/ClangSharp/sources/libClangSharp/ClangSharp.cpp:169:48: warning: 'get' is deprecated: Use cast instead [-Wdeprecated-declarations]
  169 |                     return MakeCXCursor(object.get<CompoundLiteralExpr*>(), getCursorDecl(C), getCursorTU(C));
      |                                                ^
ClangSharp/artifacts/llvm/install/include/llvm/ADT/PointerUnion.h:161:12: note: 'get' has been explicitly marked deprecated here
  161 |   inline T get() const {
      |            ^
ClangSharp/ClangSharp/sources/libClangSharp/ClangSharp.cpp:169:48: warning: 'get<clang::CompoundLiteralExpr *>' is deprecated: Use cast instead [-Wdeprecated-declarations]
  169 |                     return MakeCXCursor(object.get<CompoundLiteralExpr*>(), getCursorDecl(C), getCursorTU(C));
      |                                                ^
ClangSharp/artifacts/llvm/install/include/llvm/ADT/PointerUnion.h:160:5: note: 'get<clang::CompoundLiteralExpr *>' has been explicitly marked deprecated here
  160 |   [[deprecated("Use cast instead")]]
      |     ^
```

Author: rolfbjarne

sources/libClangSharp/ClangSharp.cpp

@@ -162,10 +162,10 @@ CXCursor clangsharp_Cursor_getArgument(CXCursor C, unsigned i) {
                 llvm::PointerUnion<BlockDecl*, CompoundLiteralExpr*> object = EWC->getObject(i);
 
                 if (isa<BlockDecl*>(object)) {
-                    return MakeCXCursor(object.get<BlockDecl*>(), getCursorTU(C));
+                    return MakeCXCursor(cast<BlockDecl*>(object), getCursorTU(C));
                 }
                 else {
-                    return MakeCXCursor(object.get<CompoundLiteralExpr*>(), getCursorDecl(C), getCursorTU(C));
+                    return MakeCXCursor(cast<CompoundLiteralExpr*>(object), getCursorDecl(C), getCursorTU(C));
                 }
             }
         }