[PATCH] Remove support for enabling / disabling multi-threading at runtime.

[Reid Kleckner]

lgtm

The commit message should mention that this had no performance impact on the clang self-host.

================
Comment at: lib/IR/Core.cpp:2709-2710
@@ +2708,4 @@
+  // time define.
+  assert(LLVMIsMultithreaded() &&
+      "Attempted to enable multi-threading mode on a non-multithreaded build.");
+  return LLVMIsMultithreaded();
----------------
The previous behavior was to return false, so we should preserve that without asserting.

http://reviews.llvm.org/D4076