How to write our own DiagnosticConsumer in clang libtooling?

Question

I want to capture the source files error and warnings when I give it as a input to my libtooling tool.

For example consider the program

main.c

int main()
{
   printf("hello");
   return 0;
} 

so clang throws warning implicitly declaring library function 'printf' with type 'int'.

How to capture these warnings in clang libtooling with DiagnosticConsumer?