Add a new check, cert-flp30-c, that diagnoses loop induction expressions of...

Add a new check, cert-flp30-c, that diagnoses loop induction expressions of floating-point type. This check corresponds to the CERT secure coding rule: https://www.securecoding.cert.org/confluence/display/c/FLP30-C.+Do+not+use+floating-point+variables+as+loop+counters

llvm-svn: 261324