Require stdcall etc parameters to be complete on ODR use

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

Differential Revision: https://reviews.llvm.org/D62975

llvm-svn: 363000