Fix X86 compilation.

This commit is to work-around an LLVM bug. LLVM's DisableUnitAtATime will
break X86 compilation (logan), and I saw this in the llvm PassManagerBuilder:
  // FIXME: We shouldn't bother with this anymore.
  MPM.add(createStripDeadPrototypesPass());

We follow the workaround above in the LLVM tree. It does magic.

Change-Id: I699757f3385a91202d34141d7f154cd0e80bae03
diff --git a/src/compiler_llvm/compilation_unit.cc b/src/compiler_llvm/compilation_unit.cc
index 30c6aa1..9e2c33a 100644
--- a/src/compiler_llvm/compilation_unit.cc
+++ b/src/compiler_llvm/compilation_unit.cc
@@ -430,8 +430,9 @@
   pm_builder.OptLevel = 3;
   pm_builder.DisableSimplifyLibCalls = 1;
   pm_builder.DisableUnitAtATime = 1;
-  pm_builder.populateModulePassManager(pm);
   pm_builder.populateFunctionPassManager(fpm);
+  pm_builder.populateModulePassManager(pm);
+  pm.add(llvm::createStripDeadPrototypesPass());
 
   // Add passes to emit ELF image
   {