[clang] Fix -Wimplicit-fallthrough in UnsafeBufferUsage.cpp [NFC]

[clang] Fix -Wimplicit-fallthrough in UnsafeBufferUsage.cpp [NFC]

/Users/jiefu/llvm-project/clang/lib/Analysis/UnsafeBufferUsage.cpp:1272:5: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
    case Strategy::Kind::Wontfix:
    ^
/Users/jiefu/llvm-project/clang/lib/Analysis/UnsafeBufferUsage.cpp:1272:5: note: insert 'LLVM_FALLTHROUGH;' to silence this warning
    case Strategy::Kind::Wontfix:
    ^
    LLVM_FALLTHROUGH;
/Users/jiefu/llvm-project/clang/lib/Analysis/UnsafeBufferUsage.cpp:1272:5: note: insert 'break;' to avoid fall-through
    case Strategy::Kind::Wontfix:
    ^
    break;
1 error generated.