[WebAssembly] Print error message for llvm.clear_cache intrinsic

Summary:
Wasm does not currently support `llvm.clear_cache` intrinsic, and this
prints a proper error message instead of segfault.

Reviewers: dschuff, sbc100, sunfish

Subscribers: jgravelle-google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64322

llvm-svn: 365731