[clang] [PowerPC][NFC] Define new alias for mma accumulate builtins (PR #147382)

[Amy Kwan via cfe-commits]

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@@ -35,6 +35,13 @@
 #define UNALIASED_CUSTOM_BUILTIN(ID, TYPES, ACCUMULATE, FEATURE)               \
   CUSTOM_BUILTIN(ID, ID, TYPES, ACCUMULATE, FEATURE)
 
+#define UNALIASED_CUSTOM_MMA_BUILTIN(ID, TYPES, FEATURE)                       \
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amy-kwan wrote:

Can we document this new `define` to explain it for someone who is not familiar with the code?

https://github.com/llvm/llvm-project/pull/147382