compiler.compiles/links: fix failure when compiling a built File object

In order to pass a File object down into the compiler impl and compile
it, we cannot pass a string with the filename, and we cannot either pass
the File object as-is, since it relies on being given Environment
attributes to calculate the relative location. So we build a fresh File
object as an absolute path.

But the code to do this was totally broken. Instead of using the File
method to get an absolute path, we used one that expected to create
builddir-relative paths... and then gave it the absolute source dir as
the "relative path portion" prefix. This worked by accident as long as
it wasn't a built File, but if it was a built file then we intentionally
didn't include that prefix -- which was wrong anyway, since we need the
build directory!

Use the correct method to get an absolute path in all cases, and emit a
warning if it was a built file. This never worked. Sometimes it crashed,
sometimes it silently returned false.

Fixes #11983

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