[clang] [WebAssembly] Add preprocessor define for half-precision (PR #90528)

[Heejin Ahn via cfe-commits]

aheejin wrote:

cc @brendandahl (I couldn't add you as a reviewer because you didn't pop up in the reviewers list)

Also, this just adds the preprocessor directive, but I'm wondering whether you really meant to add this to bleeding-edge: https://github.com/llvm/llvm-project/commit/d9fd0ddef38bb9d5cce7300ff820272183c09fcd#r141495634

https://github.com/llvm/llvm-project/pull/90528