[Support] Fix build under Emscripten

Summary:
Emscripten's libc doesn't define MNT_LOCAL, thus causing a build
failure in the fallback path. However, to the best of my knowledge,
it also doesn't support remote file system mounts, so we may simply
return `true` here (as we do for e.g. Fuchsia). With this fix, the
core LLVM libraries build correctly under emscripten (though some
of the tools and utils do not).

Reviewers: kripken
Differential Revision: https://reviews.llvm.org/D63688

llvm-svn: 364143

llvm/lib/Support/Unix/Path.inc

@@ -424,6 +424,9 @@ static bool is_local_impl(struct STATVFS &Vfs) {
 #elif defined(__Fuchsia__)
   // Fuchsia doesn't yet support remote filesystem mounts.
   return true;
+#elif defined(__EMSCRIPTEN__)
+  // Emscripten doesn't currently support remote filesystem mounts.
+  return true;
 #elif defined(__HAIKU__)
   // Haiku doesn't expose this information.
   return false;