[WebAssembly] Rename except_ref type to exnref

Summary:
We agreed to rename `except_ref` to `exnref` for consistency with other
reference types in
WebAssembly/exception-handling#79. This also
renames WebAssemblyInstrExceptRef.td to WebAssemblyInstrRef.td in order
to use the file for other reference types in future.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64703

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@366145 91177308-0d34-0410-b5e6-96231b3b80d8

Author: aheejin

wasm/WriterUtils.cpp

@@ -182,8 +182,8 @@ std::string lld::toString(ValType type) {
     return "f64";
   case ValType::V128:
     return "v128";
-  case ValType::EXCEPT_REF:
-    return "except_ref";
+  case ValType::EXNREF:
+    return "exnref";
   }
   llvm_unreachable("Invalid wasm::ValType");
 }