[clang][ASTImporter] Support Importer of BuiltinBitCastExpr (#74813)

Since import `ExplicitCastExpr` lacks of processing
`BuiltinBitCastExprClass` type, it would reach to the 'unreachable' code
and produce the crash. This patch aims to fix the
[crash](https://github.com/llvm/llvm-project/issues/74774

) and try to
handle `BuiltinBitCastExpr`.

Co-authored-by: huqizhi <836744285@qq.com>