Further fixes when thiscall is the default for methods.

The previous patches tried to deduce the correct function type. I now realize
this is not possible in general. Consider

class foo {
    template <typename T> static void bar(T v);
};
extern template void foo::bar(const void *);

We will only know that bar is static after a lookup, so we have to handle this
in the template instantiation code.

This patch reverts my previous two changes (but not the tests) and instead
handles the issue in DeduceTemplateArguments.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195154 91177308-0d34-0410-b5e6-96231b3b80d8