[analyzer] PR41185: Fix regression where __builtin_* functions weren't recognized

For the following code snippet:

void builtin_function_call_crash_fixes(char *c) {
  __builtin_strncpy(c, "", 6);
  __builtin_memset(c, '\0', (0));
  __builtin_memcpy(c, c, 0);
}
security.insecureAPI.DeprecatedOrUnsafeBufferHandling caused a regression, as it
didn't recognize functions starting with __builtin_. Fixed exactly that.

I wanted to modify an existing test file, but the two I found didn't seem like
perfect candidates. While I was there, I prettified their RUN: lines.

Differential Revision: https://reviews.llvm.org/D59812

llvm-svn: 358609