Revert "[analyzer; alternate edges] don't add an edge incoming from the start of a function"

...and make this work correctly in the current codebase.

After living on this for a while, it turns out to look very strange for
inlined functions that have only a single statement, and somewhat strange
for inlined functions in general (since they are still conceptually in the
middle of the path, and there is a function-entry path note).

It's worth noting that this only affects inlined functions; in the new
arrow generation algorithm, the top-level function still starts at the
first real statement in the function body, not the enclosing CompoundStmt.

This reverts r182078 / dbfa950abe0e55b173286a306ee620eff5f72ea.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182963 91177308-0d34-0410-b5e6-96231b3b80d8