In fast-completion, a function body can be replaced with another function
body parsed from a new buffer. In such cases, during typechecking the
expressions in the *new* function body, a source location range check in
UnqualifiedLookup didn't work well because they are not from the same
buffer.
This patch workaround it by skipping the source range checks and returns
'success' in such cases.
rdar://problem/58881999
ff2ccd485c