[OptDiag, opt-viewer] Save callee's location and display as link

With this we get a new field in the YAML record if the value being
streamed out has a debug location.  For examples, please see the changes
to the tests.

This is then used in opt-viewer to display a link for the callee
function in the inlining remarks.

Differential Revision: https://reviews.llvm.org/D26366

llvm-svn: 286169