[clang] [libc++] Add builtin to clear padding bytes (prework for P0528R3) (PR #75371)

[Louis Dionne via cfe-commits]

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@@ -0,0 +1,556 @@
+// RUN: mkdir -p %t
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ldionne wrote:

Are these files missing a license?

https://github.com/llvm/llvm-project/pull/75371