The Go frontend passes closures through to functions using the
functions __go_set_closure and __go_get_closure.  The
expectation is that there are no function calls between
set_closure and get_closure.  However, it turns out that there
can be function calls if some of the function arguments
require type conversion to an interface type.  Converting to
an interface type can allocate memory, and that can in turn
trigger a garbage collection, and that can in turn call pool
cleanup functions that may call __go_set_closure.  So the
called function can see the wrong closure value, which is bad.

This patch fixes the problem in two different ways.  First, we
move all type conversions in function arguments into temporary
variables so that they can not appear before the call to
__go_set_closure.  (This required shifting the flatten phase
after the simplify_thunk phase, since the latter expects to
work with unconverted argument types.)  Second, we fix the
memory allocation function to preserve the closure value
across any possible garbage collection.

A test case is the libgo database/sql check run with the
environment variable GOGC set to 1.

From-SVN: r213932