[Fuchsia] Never link in implicit "system dependencies" of sanitizer runtimes

[mcgrathr]

Summary

This is never appropriate on Fuchsia and any future needs for
system library dependencies of compiler-supplied runtimes will
be addressed via .deplibs instead of driver hacks.

[phosek]

LGTM % nit

clang/lib/Driver/ToolChains/CommonArgs.cpp

586

Nit: LLVM style says no braces for blocks with a single statement.

[mcgrathr]

builds now.

[mcgrathr]

style nit

clang/lib/Driver/ToolChains/CommonArgs.cpp

if (llvm::sys::fs::exists(SanRT + ".syms")) {
     CmdArgs.push_back(Args.MakeArgString("--dynamic-list=" + SanRT + ".syms"));
     return true;
   }
   return false;
 }
 
 void tools::linkSanitizerRuntimeDeps(const ToolChain &TC,
                                      ArgStringList &CmdArgs) {
+  // Fuchsia never needs these.  Any sanitizer runtimes with system
+  // dependencies use the `.deplibs` feature instead.
+  if (TC.getTriple().isOSFuchsia())
+    return;
+
   // Force linking against the system libraries sanitizers depends on
   // (see PR15823 why this is necessary).
   CmdArgs.push_back("--no-as-needed");
   // There's no libpthread or librt on RTEMS & Android.
   if (TC.getTriple().getOS() != llvm::Triple::RTEMS &&
       !TC.getTriple().isAndroid()) {
     CmdArgs.push_back("-lpthread");
     if (!TC.getTriple().isOSOpenBSD())