[clang] [CIR] Enable x86_64 calling-convention lowering by default (PR #215026)

[Andy Kaylor via cfe-commits]

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@@ -16,13 +16,21 @@ void f2(void) {
   f1(s);
 }
 
+// S is one eightbyte of INTEGER class, so it is passed as one i64.
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andykaylor wrote:

Comments like this aren't needed.

https://github.com/llvm/llvm-project/pull/215026