Simplify some CodeGenTypes::arrangeXXX functions. No functionality change

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215606 91177308-0d34-0410-b5e6-96231b3b80d8
clang/test/Modules/cxx-irgen.cpp: Let it tolerant of x86_thiscallcc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215607 91177308-0d34-0410-b5e6-96231b3b80d8
Sema: Permit nullptr template args in MSVC compat mode

This fixes a regression I caused back in r211766.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215609 91177308-0d34-0410-b5e6-96231b3b80d8