Fixed a problem that caused LLDB to fail to execute

expressions that used function pointers.  The problem
was that IRForTarget previously only scanned the IR
for the expression for call instructions; if a function
was used in another context, it was ignored.

Now LLDB scans the Module for functions that are only
declared (not also defined -- so these are externals);
it then constructs function pointers for these
functions and substitutes them wherever the function
is used.

Also made some changes so that "expr main" works just
as well as "expr &main"; they end up being the same
code, but LLDB was generating the result variable in
different ways.

llvm-svn: 136928