When recursively instantiating function templates, keep track of the

instantiation stack so that we provide a full instantiation
backtrace. Previously, we performed all of the instantiations implied
by the recursion, but each looked like a "top-level" instantiation.

The included test case tests the previous fix for the instantiation of
DeclRefExprs. Note that the "instantiated from" diagnostics still
don't tell us which template arguments we're instantiating with.

llvm-svn: 74540