[llgo] llgoi: separate evaluation from printing

Summary:
Separate the evaluation of expressions from printing
of results. This is in preparation for splitting the
core of the interpreter out for use in alternative
interpreter frontends.

At the same time, the output is made less noisy in
response to comments on the golang-nuts announcement.
We would ideally print out values using Go syntax,
but this is impractical until we have libgo based on
Go 1.5. When that happens, fmt's %#v will handle
reflect.Value better, and so we can fix/filter type
names to remove automatically generated package names.

Reviewers: pcc

Subscribers: llvm-commits, axw

Differential Revision: http://reviews.llvm.org/D13761

llvm-svn: 267374