[LTT] mark the CFI jumptable naked on Windows (#170371)

We were not marking the `.cfi.jumptable`​ functions as `naked`​ on windows. The referenced bug (https://llvm.org/bugs/show_bug.cgi?id=28641#c3) appears to be fixed:

```bash
build/bin/opt -S -passes=lowertypetests -mtriple=i686-pc-win32 llvm/test/Transforms/LowerTypeTests/function.ll | build/bin/llc -O0
```

```
L_.cfi.jumptable:                       # @.cfi.jumptable
# %bb.0:                                # %entry
        #APP
        jmp     _f.cfi@PLT
        int3
        int3
        int3

        #NO_APP
        #APP
        jmp     _g.cfi@PLT
        int3
        int3
        int3

        #NO_APP
                                        # -- End function
        .section        .rdata,"dr"
        .p2align        4, 0x0                          # @0

```

Not seeing the spilled registers described in the bug anymore.

Author: mtrofin

llvm/lib/Transforms/IPO/LowerTypeTests.cpp

@@ -1554,12 +1554,7 @@ void LowerTypeTestsModule::createJumpTable(
 
   // Align the whole table by entry size.
   F->setAlignment(Align(getJumpTableEntrySize(JumpTableArch)));
-  // Skip prologue.
-  // Disabled on win32 due to https://llvm.org/bugs/show_bug.cgi?id=28641#c3.
-  // Luckily, this function does not get any prologue even without the
-  // attribute.
-  if (OS != Triple::Win32)
-    F->addFnAttr(Attribute::Naked);
+  F->addFnAttr(Attribute::Naked);
   if (JumpTableArch == Triple::arm)
     F->addFnAttr("target-features", "-thumb-mode");
   if (JumpTableArch == Triple::thumb) {

llvm/test/Transforms/LowerTypeTests/function.ll

@@ -1,7 +1,7 @@
-; RUN: opt -S -passes=lowertypetests -mtriple=i686-unknown-linux-gnu %s | FileCheck --check-prefixes=X86,X86-LINUX,NATIVE,JT8 %s
-; RUN: opt -S -passes=lowertypetests -mtriple=x86_64-unknown-linux-gnu %s | FileCheck --check-prefixes=X86,X86-LINUX,NATIVE,JT8 %s
-; RUN: opt -S -passes=lowertypetests -mtriple=i686-pc-win32 %s | FileCheck --check-prefixes=X86,X86-WIN32,NATIVE,JT8 %s
-; RUN: opt -S -passes=lowertypetests -mtriple=x86_64-pc-win32 %s | FileCheck --check-prefixes=X86,X86-WIN32,NATIVE,JT8 %s
+; RUN: opt -S -passes=lowertypetests -mtriple=i686-unknown-linux-gnu %s | FileCheck --check-prefixes=X86,NATIVE,JT8 %s
+; RUN: opt -S -passes=lowertypetests -mtriple=x86_64-unknown-linux-gnu %s | FileCheck --check-prefixes=X86,NATIVE,JT8 %s
+; RUN: opt -S -passes=lowertypetests -mtriple=i686-pc-win32 %s | FileCheck --check-prefixes=X86,NATIVE,JT8 %s
+; RUN: opt -S -passes=lowertypetests -mtriple=x86_64-pc-win32 %s | FileCheck --check-prefixes=X86,NATIVE,JT8 %s
 ; RUN: opt -S -passes=lowertypetests -mtriple=riscv32-unknown-linux-gnu %s | FileCheck --check-prefixes=RISCV,NATIVE,JT8 %s
 ; RUN: opt -S -passes=lowertypetests -mtriple=riscv64-unknown-linux-gnu %s | FileCheck --check-prefixes=RISCV,NATIVE,JT8 %s
 ; RUN: opt -S -passes=lowertypetests -mtriple=wasm32-unknown-unknown %s | FileCheck --check-prefix=WASM32 %s
@@ -114,8 +114,7 @@ define i1 @foo(ptr %p) {
 
 ; NATIVE-SAME: "s"(ptr @g.cfi)
 
-; X86-LINUX: attributes #[[ATTR]] = { naked nocf_check noinline }
-; X86-WIN32: attributes #[[ATTR]] = { nocf_check noinline }
+; X86: attributes #[[ATTR]] = { naked nocf_check noinline }
 ; ARM: attributes #[[ATTR]] = { naked noinline
 ; THUMB: attributes #[[ATTR]] = { naked noinline "target-cpu"="cortex-a8" "target-features"="+thumb-mode" }
 ; THUMBV6M: attributes #[[ATTR]] = { naked noinline "target-features"="+thumb-mode" }