[clang] [Clang] add ext warning for missing return in 'main' for C89 mode (PR #134617)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,6 @@
+/* RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -pedantic -Wno-strict-prototypes %s
+ */
+
+int main() {
----------------
AaronBallman wrote:

You should also have a test where `main` does return a value to show that we silence the diagnostic, and another test along the lines of:
```
int main() {
  if (1)
    exit(1);
}
```
which should also silence the diagnostic.

https://github.com/llvm/llvm-project/pull/134617