Hitherto the IRForTarget infrastructure has mainly
been suitable for preparing a single IR function
for operation in the target.  However, using blocks
and lambdas creates other IR functions that also
need to be processed.

I have audited IRForTarget to make it process
multiple functions.  Where IRForTarget would add
new instructions at the beginning of the main
expression function, it now adds them on-demand
in the function where they are needed.  This is
enabled by a system of FunctionValueCaches, which
invoke a lambda to create or derive the values as
needed, or report the result of that lambda if it
has already been called for the given function.

<rdar://problem/14180236>