Functions using stdcall, fastcall, or vectorcall with C linkage mangle
in the size of the parameter pack. Calculating the size of the pack
requires the parameter types to complete, which may require template
instantiation.
Previously, we would crash during IRgen when requesting the size of
incomplete or uninstantiated types, as in this reduced example:
struct Foo; void __fastcall bar(struct Foo o); void (__fastcall *fp)(struct Foo) = &bar;
Reported in Chromium here: https://crbug.com/971245
Do we need those checks or would it be enough to just check the calling convention?
Also, I think s/Do nothing/return false/