[clang] [clang][HIP] Normalize __cdecl on abstract function types to the device default CC on amdgcnspirv (PR #210882)

[Tom Honermann via cfe-commits]

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@@ -172,8 +200,14 @@ class LLVM_LIBRARY_VISIBILITY BaseSPIRTargetInfo : public TargetInfo {
                                                             : CCCR_Warning;
   }
 
-  CallingConv getDefaultCallingConv() const override {
-    return CC_SpirFunction;
+  CallingConv getDefaultCallingConv() const override { return CC_C; }
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tahonermann wrote:

Since the default calling convention returned by `TargetInfo::getDefaultCallingConv()` is `CC_C`, this function can be removed; that is what every other target (except X86 for some reason) does.

https://github.com/llvm/llvm-project/pull/210882