bugprone-argument-comment: SourceLocation valid judgment avoid emitting...

bugprone-argument-comment: SourceLocation valid judgment avoid emitting coredump in isInSystemHeader

If the Node has an invalid location, it will trigger assert in
isInSystemHeader(...).

void test() {
  __builtin_va_list __args;
  // __builtin_va_list has no defination in any source file and its
  // CXXConstructorDecl has invalid sourcelocation
}
coredump with "Assertion `Loc.isValid() && "Can't get file
characteristic of invalid loc!"' failed." in
getFileCharacteristic(SourceLocation).