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

[Fangrui Song via cfe-commits]

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@@ -0,0 +1,11 @@
+// RUN: %clang -c %s -### 2>&1 | FileCheck -check-prefix T0 %s
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MaskRay wrote:

Test subdirectories are added conservatively. The main use cases are target-specific tests and flang. `Driver/XRay` is somehow an unneeded use that you can ignore.
```
% echo {CodeGen,Sema,Driver}/*(/)
CodeGen/aarch64_neon_sve_bridge_intrinsics CodeGen/aarch64-sme-intrinsics CodeGen/aarch64-sve2-intrinsics CodeGen/aarch64-sve2p1-intrinsics CodeGen/aarch64-sve-intrinsics CodeGen/arc CodeGen/arm-mve-intrinsics CodeGen/assignment-tracking CodeGen/avr CodeGen/CSKY CodeGen/Inputs CodeGen/LoongArch CodeGen/PowerPC CodeGen/RISCV CodeGen/SystemZ CodeGen/VE CodeGen/WebAssembly CodeGen/X86 Sema/aarch64-sme-intrinsics Sema/aarch64-sve2-intrinsics Sema/aarch64-sve2p1-intrinsics Sema/aarch64-sve-intrinsics Sema/Inputs Driver/flang Driver/Inputs Driver/XRay
```

FWIW you can do

* `path/bin/llvm-lit clang/test/Driver/bounds-safety-*`
* `LIT_OPTS=--filter=bounds-safety path/bin/llvm-lit clang/test/Driver`

and even create an alias for `LIT_OPTS=--filter=bounds-safety `

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