[coro] Preserve scope line for compiler generated functions

Coro-split functions with an active suspend point have their scope line set to
the line of the suspend point. However for compiler generated functions, this
results in debug info with unconventional results: a file named
`<compiler-generated>` with a non-zero line number. The convention for
`<compiler-generated>` is that the line number is zero.

This change propagates the scope line only for non-compiler generated
functions.

Differential Revision: https://reviews.llvm.org/D102412