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

[Yeoul Na via cfe-commits]

================
@@ -0,0 +1,12 @@
+// This reports a warning to follow the default behavior of ClangAs.
+// RUN: %clang -fexperimental-bounds-safety -x assembler -c %s -o /dev/null 2>&1 | FileCheck -check-prefix WARN %s
----------------
rapidsna wrote:

I think `-x assembler` and `-x assembler-with-cpp` are more specific and should cover what I want. 

`.s` must be `-x assembler` (except for Darwin which does `-x assembler-with-cpp` for `.s`), but that specific behavior should be tested separately but not for this PR.

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