[clang] [PowerPC] Update altivec.h to use __inline__ for c89 compatibility (PR #134430)

[Hubert Tong via cfe-commits]

================
@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -o - %s | FileCheck %s
 // RUN: %clang_cc1 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -flax-vector-conversions=none -o - %s | FileCheck %s
 // RUN: %clang_cc1 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -x c++ -o - %s | FileCheck %s
+// RUN: %clang_cc1 -std=c89 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -o - %s | FileCheck %s
----------------
hubert-reinterpretcast wrote:

Maybe better to keep all the C cases together.
```suggestion
// RUN: %clang_cc1 -std=c89 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -o - %s | FileCheck %s
// RUN: %clang_cc1 -triple powerpc64-unknown-unknown -target-feature +altivec -ffreestanding -emit-llvm -x c++ -o - %s | FileCheck %s
```

https://github.com/llvm/llvm-project/pull/134430