[clang] [clang][PowerPC] Add flag to enable compatibility with GNU for complex arguments (PR #77732)

[Nemanja Ivanovic via cfe-commits]

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@@ -78,6 +78,13 @@ class CodeGenOptions : public CodeGenOptionsBase {
     SRCK_InRegs    // Small structs in registers (-freg-struct-return).
   };
 
+  enum ComplexArgumentConventionKind {
+    CMPLX_Default,
+    CMPLX_OnStack,
+    CMPLX_OnGPR, // if ppc32 -fcomplex-ppc-gnu-abi
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nemanjai wrote:

Nit: can we use `In[GF]PR` in the names as "in registers"/"on stack" are in common usage.

https://github.com/llvm/llvm-project/pull/77732