Rename wasm-delegations[-fields].h to def files (NFC) (#3941)

These files are special in that they use define symbols that are not
defined within those files or other files included in those files; they
are supposed to be defined in source files that include these headers.
This has caused clang-tidy to fail every time these files have changed
because they are not compilable per se.

This PR solves the problem by changing their extension to `def`, which
is also used in LLVM codebase. LLVM has dozens of files like this whose
extension is `def`, which makes these not checked by clang-tidy.

0 files changed, 0 insertions, 0 deletions