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

[Nick Desaulniers via cfe-commits]

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+// RUN: %clang -c %s -### 2>&1 | FileCheck -check-prefix T0 %s
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nickdesaulniers wrote:

> And make it easier to run bounds safety related tests only.

> Provides a very convenient way to run only -fbounds-safety tests.

Then give each test a similar function name prefix, and then you can do `llvm-lit -vv llvm/test/Driver/fbounds-safety-*`, as @MaskRay said.  Don't start creating new directory structures that don't exist for every other feature implemented by the compiler.

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