[WebAssembly] Fix type names in truncation builtins

Summary: Use the same convention as all the other WebAssembly builtin names.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, sunfish, kristina, cfe-commits

Differential Revision: https://reviews.llvm.org/D53724

llvm-svn: 345804