[MinGW] Automatically mangle Windows-specific entry points as C

[mstorsjo]

Summary

This mangles entry points wmain, WinMain, wWinMain or DllMain as C functions, to match the ABI for these functions.

We already did the same for these functions in MSVC mode, but we also should do the same in the Itanium ABI.

This fixes PR38124.

[majnemer]

LGTM

cfe/trunk/lib/AST/ItaniumMangle.cpp

if (FD) {
     // Overloadable functions need mangling.
     if (FD->hasAttr<OverloadableAttr>())
       return true;
 
     // "main" is not mangled.
     if (FD->isMain())
       return false;
 
+    // The Windows ABI expects that we would never mangle "typical"
+    // user-defined entry points regardless of visibility or freestanding-ness.
+    //
+    // N.B. This is distinct from asking about "main".  "main" has a lot of
+    // special rules associated with it in the standard while these
+    // user-defined entry points are outside of the purview of the standard.
+    // For example, there can be only one definition for "main" in a standards
+    // compliant program; however nothing forbids the existence of wmain and
+    // WinMain in the same translation unit.
+    if (FD->isMSVCRTEntryPoint())
+      return false;
+
     // C++ functions and those whose names are not a simple identifier need
     // mangling.
     if (!FD->getDeclName().isIdentifier() || L == CXXLanguageLinkage)
       return true;
 
     // C functions are not mangled.
     if (L == CLanguageLinkage)
       return false;

cfe/trunk/test/CodeGenCXX/mangle-mingw.cpp

+// RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-w64-mingw32 | FileCheck %s
+
+int func() { return 0; }
+// CHECK-DAG: @_Z4funcv
+
+int main() { return 0; }
+// CHECK-DAG: @main
+
+int wmain() { return 0; }
+// CHECK-DAG: @wmain
+
+int WinMain() { return 0; }
+// CHECK-DAG: @WinMain
+
+int wWinMain() { return 0; }
+// CHECK-DAG: @wWinMain
+
+int DllMain() { return 0; }
+// CHECK-DAG: @DllMain