Improve diagnostic checking for va_start to also warn on other instances of...

Improve diagnostic checking for va_start to also warn on other instances of undefined behavior, such as a parameter declared with the register keyword in C, or a parameter of a type that undergoes default argument promotion.

This helps cover some more of the CERT secure coding rule EXP58-CPP. Pass an object of the correct type to va_start (https://www.securecoding.cert.org/confluence/display/cplusplus/EXP58-CPP.+Pass+an+object+of+the+correct+type+to+va_start).

llvm-svn: 267338