[NFC] [Serialization] Reordering lexcical and visible TU block after type decl offsets

This patch reorder the lexical block for the translation unit, visible update block for the TU and
the viisble upaete block for the extern C context after the type decl
offsets block.

This should be a NFC patch.

This is helpful for later optimizations for eliding unreachable
declarations in the global module fragment. See the comments in
https://github.com/llvm/llvm-project/pull/76930.

Simply, if we want to get the reachable sets of declaratins during the
writing process, we need to write the file-level context later than the
process of writing declarations (which is the main process to determine
the reachable set).