[analyzer] Fix diagnostics in callees of interesting callees.

removeUnneededCalls() is responsible for removing path diagnostic pieces within
functions that don't contain "interesting" events. It makes bug reports
much tidier.

When a stack frame is known to be interesting, the function doesn't descend
into it to prune anything within it, even other callees that are totally boring.

Fix the function to prune boring callees in interesting stack frames.

Differential Revision: https://reviews.llvm.org/D45117

llvm-svn: 329102