Don't claim that va_start has special semantic checks

We don't have special checks for BI_va_start in
Sema::CheckBuiltinFunctionCall, so setting the 't' flag for va_start in
Builtins.def disables semantic checking for it. That's not desired, and
IRGen crashes when it tries to generate a call to va_start that doesn't
have at least one argument.

Follow-up to r322573

Fixes PR36565

llvm-svn: 326622