Windows ARM: ignore calling conventions as described on MSDN

Summary:
MSDN says that fastcall, stdcall, thiscall, and vectorcall are all
accepted but ignored on ARM and X64.
https://msdn.microsoft.com/en-us/library/984x0h58.aspx

MSDN also says cdecl is also accepted and typically ignored
This patch brings ARM in line with how we ignore them for X64

Reviewers: rnk

Subscribers: compnerd, cfe-commits

Differential Revision: http://reviews.llvm.org/D12034

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245076 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Basic/Targets.cpp

@@ -4959,6 +4959,19 @@ public:
   BuiltinVaListKind getBuiltinVaListKind() const override {
     return TargetInfo::CharPtrBuiltinVaList;
   }
+  CallingConvCheckResult checkCallingConvention(CallingConv CC) const override {
+    switch (CC) {
+    case CC_X86StdCall:
+    case CC_X86ThisCall:
+    case CC_X86FastCall:
+    case CC_X86VectorCall:
+      return CCCR_Ignore;
+    case CC_C:
+      return CCCR_OK;
+    default:
+      return CCCR_Warning;
+    }
+  }
 };
 
 // Windows ARM + Itanium C++ ABI Target

test/Parser/arm-windows-calling-convention-handling.c

@@ -1,10 +1,9 @@
 // RUN: %clang_cc1 -triple thumbv7-windows -fms-compatibility -fsyntax-only -verify %s
 
-int __cdecl cdecl(int a, int b, int c, int d) { // expected-warning {{calling convention '__cdecl' ignored for this target}}
+int __cdecl cdecl(int a, int b, int c, int d) { // expected-no-diagnostics
   return a + b + c + d;
 }
 
-float __stdcall stdcall(float a, float b, float c, float d) { // expected-warning {{calling convention '__stdcall' ignored for this target}}
+float __stdcall stdcall(float a, float b, float c, float d) { // expected-no-diagnostics
   return a + b + c + d;
 }
-

test/Parser/x64-windows-calling-convention-handling.c

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -triple x86_64-windows -fms-compatibility -fsyntax-only -verify %s
+
+int __cdecl cdecl(int a, int b, int c, int d) { // expected-no-diagnostics
+  return a + b + c + d;
+}
+
+float __stdcall stdcall(float a, float b, float c, float d) { // expected-no-diagnostics
+  return a + b + c + d;
+}