Win64: set HasWinCFI for naked functions

[TimNN]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=32912.

HasWinCFI is normally set during prolog insertion, since that doesn't happen for naked functions it remains unset for those which causes assertion errors later.

[TimNN]

Another fix has been commited.

lib/CodeGen/PrologEpilogInserter.cpp

bool PEI::runOnMachineFunction(MachineFunction &Fn) {
 
   // Add prolog and epilog code to the function.  This function is required
   // to align the stack frame as necessary for any stack variables or
   // called functions.  Because of this, calculateCalleeSavedRegisters()
   // must be called before this function in order to set the AdjustsStack
   // and MaxCallFrameSize variables.
   if (!F->hasFnAttribute(Attribute::Naked))
     insertPrologEpilogCode(Fn);
+  else
+    // Normally HasWinCFI would be set sometime during prolog insertion, since
+    // that doesn't happen insert it here since later code depends on it being
+    // set.
+    Fn.setHasWinCFI(false);
 
   // Replace all MO_FrameIndex operands with physical register references
   // and actual offsets.
   //
   replaceFrameIndices(Fn);
 
   // If register scavenging is needed, as we've enabled doing it as a
   // post-pass, scavenge the virtual registers that frame index elimination

test/CodeGen/X86/naked-win-x86_64.ll

+; RUN: llc < %s
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+define internal void @naked_fn() naked {
+start:
+  ret void
+}