[WebAssembly] Add mutable globals feature

Summary:
This feature is not actually used for anything in the WebAssembly
backend, but adding it allows users to get it into the target features
sections of their objects, which makes these objects
future-compatible.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, jdoerfert, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D60013

llvm-svn: 357321
diff --git a/clang/lib/Basic/Targets/WebAssembly.h b/clang/lib/Basic/Targets/WebAssembly.h
index efd32c7..a0516da 100644
--- a/clang/lib/Basic/Targets/WebAssembly.h
+++ b/clang/lib/Basic/Targets/WebAssembly.h
@@ -35,6 +35,7 @@
   bool HasExceptionHandling = false;
   bool HasBulkMemory = false;
   bool HasAtomics = false;
+  bool HasMutableGlobals = false;
 
 public:
   explicit WebAssemblyTargetInfo(const llvm::Triple &T, const TargetOptions &)