llgo: Get function literals/closures working once more
Various fixes to work with Go 1.1

There are some new stub functions for pkg/reflect.
llgo: Represent functions as a pair of pointers

This commit changes function representation to be a
pair of pointers: (fnptr, context). The first pointer
is a raw function pointer; the second pointer is a
"context" pointer, which is non-nil for closures. For
a plain old function, the function value will be
constant, with the second value being const-null.

In addition, "receiver" has been removed from LLVMValue,
and we overload this pair-of-pointers representation to
enable passing bound methods around internally. In this
case, the second item in the pair may have some arbitrary
non-pointer type (i.e. the type of the method receiver).

Closures no longer use trampolines, instead setting the
function pair's context value directly.

The most complex change is in VisitCallExpr: to avoid
generating additional code for plain old functions, it
first checks if the context value is const-null, and if
so, calls the function pointer as it previously would
have. If the context is not const-null, then code is
generated to check if it is non-null at runtime, and if
so, call the function with an additional argument
(the context) prepended to the argument list; otherwise
call the function pointer as it previoulst would have.

There are some problems remaining with the runtime
where functions are assumed to occupy one word, and
also in the use of intrinsics (namely llvm.trap).