[WebAssembly] Support Leak Sanitizer on Emscripten

[quantum]

Summary

LSan is currently being ported to Emscripten and mostly works.

Enabling the support in upstream would simplify testing.

[aheejin]

Oh, can we have a test case?

[quantum]

LSan is supposed to be linked in to replace some C library functions, and does not change the generated LLVM IR code, so there is nothing to actually test in LLVM.

[aheejin]

I see. Do you want me to commit this, or you are gonna get a commit access?

[quantum]

It's not urgent, so I am willing to wait a day or two for commit access. If I still can't get access by then, I'll ask someone else to commit this.

clang/lib/Driver/ToolChains/WebAssembly.cpp

void WebAssembly::AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
   case ToolChain::CST_Libstdcxx:
     llvm_unreachable("invalid stdlib name");
   }
 }
 
 SanitizerMask WebAssembly::getSupportedSanitizers() const {
   SanitizerMask Res = ToolChain::getSupportedSanitizers();
   if (getTriple().isOSEmscripten()) {
-    Res |= SanitizerKind::Vptr;
+    Res |= SanitizerKind::Vptr | SanitizerKind::Leak;
   }
   return Res;
 }
 
 Tool *WebAssembly::buildLinker() const {
   return new tools::wasm::Linker(*this);
 }