Re-land "MS ABI: lambda call operators are instance methods and should use thiscall"

Re-land "MS ABI: lambda call operators are instance methods and should use thiscall"

Update the test cases to pass when lambda call operators use thiscall.

Update the lambda-to-block conversion operator to use the default free
function calling convention instead of the call operator's convention.

This reverts commit r233082 and re-instates r233023.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@233835 91177308-0d34-0410-b5e6-96231b3b80d8