Fix a few compiler warnings.

[rolfbjarne]

• Use 'cast' instead of 'get'.

  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")]]
        |     ^
  

• Use 'isa' instead of 'is'.

  Fixes:

  ClangSharp/ClangSharp/sources/libClangSharp/ClangSharp.cpp:165:28: warning: 'is' is deprecated: Use isa instead [-Wdeprecated-declarations]
    165 |                 if (object.is<BlockDecl*>()) {
        |                            ^
  ClangSharp/artifacts/llvm/install/include/llvm/ADT/PointerUnion.h:152:15: note: 'is' has been explicitly marked deprecated here
    152 |   inline bool is() const {
        |               ^
  ClangSharp/ClangSharp/sources/libClangSharp/ClangSharp.cpp:165:28: warning: 'is<clang::BlockDecl *>' is deprecated: Use isa instead [-Wdeprecated-declarations]
    165 |                 if (object.is<BlockDecl*>()) {
        |                            ^
  ClangSharp/artifacts/llvm/install/include/llvm/ADT/PointerUnion.h:151:5: note: 'is<clang::BlockDecl *>' has been explicitly marked deprecated here
    151 |   [[deprecated("Use isa instead")]]
        |     ^