[WebAssembly] Make driver -pthread imply linker --shared-memory

Summary:
This eliminates a linker error the user might otherwise see about how
using the 'atomics' feature requires --shared-memory.

Reviewers: sbc100, aheejin

Subscribers: dschuff, jgravelle-google, sunfish, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59721

llvm-svn: 356817
diff --git a/clang/lib/Driver/ToolChains/WebAssembly.cpp b/clang/lib/Driver/ToolChains/WebAssembly.cpp
index 0104cbb..b69588d 100644
--- a/clang/lib/Driver/ToolChains/WebAssembly.cpp
+++ b/clang/lib/Driver/ToolChains/WebAssembly.cpp
@@ -62,8 +62,10 @@
     if (ToolChain.ShouldLinkCXXStdlib(Args))
       ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
 
-    if (Args.hasArg(options::OPT_pthread))
+    if (Args.hasArg(options::OPT_pthread)) {
       CmdArgs.push_back("-lpthread");
+      CmdArgs.push_back("--shared-memory");
+    }
 
     CmdArgs.push_back("-lc");
     AddRunTimeLibs(ToolChain, ToolChain.getDriver(), CmdArgs, Args);