[WebAssembly] Add --gc-sections to the link line.

This will eventually be accompanied with a change to enable -ffunction-sections
and -fdata-sections by default, which is currently delayed by some development
process issues.

llvm-svn: 256967
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 8468105..598ebc9 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -6534,6 +6534,11 @@
   ArgStringList CmdArgs;
   CmdArgs.push_back("-flavor");
   CmdArgs.push_back("ld");
+
+  // Enable garbage collection of unused input sections by default, since code
+  // size is of particular importance.
+  CmdArgs.push_back("--gc-sections");
+
   AddLinkerInputs(getToolChain(), Inputs, Args, CmdArgs);
   CmdArgs.push_back("-o");
   CmdArgs.push_back(Output.getFilename());