[clang] [-Wunsafe-buffer-usage] Fix a bug and suppress libc warnings for C files (PR #109496)

[via cfe-commits]

================
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -Wunsafe-buffer-usage %s -verify %s
----------------
jkorous-apple wrote:

FWIW the language mode is set via `-x` command line option. It is only inferred from the file extension if it's missing.

https://github.com/llvm/llvm-project/pull/109496