[clang] [Driver][BoundsSafety] Add -fbounds-safety-experimental flag (PR #70480)

[Fangrui Song via cfe-commits]

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+; RUN: %clang -fbounds-safety-experimental -x ir -S %s -o /dev/null 2>&1 | FileCheck %s
+; RUN: %clang_cc1 -fbounds-safety-experimental -x ir -S %s -o /dev/null 2>&1 | FileCheck %s
+
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MaskRay wrote:

We usually test `%clang` for driver and `%clang_cc1` for the others. Also, if we test `%clang_cc1`, we omit `%clang`.

https://github.com/llvm/llvm-project/pull/70480