[WebAssembly] Add WASM_SEC_LAST_KNOWN to BinaryFormat section types list [NFC]

There are 3 places where we were using WASM_SEC_TAG as the "last" known
section type, which requires updating (or leaves a bug) when a new known
section type is added. Instead add a "last type" to the enum for this
purpose.

Differential Revision: https://reviews.llvm.org/D127164