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

[Eli Friedman via cfe-commits]

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@@ -0,0 +1,556 @@
+// RUN: mkdir -p %t
+// RUN: %clang++ %s -o %t/run
+// RUN: %t/run
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efriedma-quic wrote:

Tests that need to actually run generated code go into the test-suite repo.  (I don't think that's really necessary here, though; if you're going to start using this in libc++, libc++ runtime tests will give us equivalent test coverage.)

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