Fixes the filescope test.

When we moved to using the Debug data for determining filescope (a much
better approach), the old test was broken and had to be fixed.

Fixes "within-file" scoping for Policy files.

When multiple files are links with llvm-link calls to
getSourceFilename() return "llvm-link" as the source. This change
properly uses the debug info to the original source filename.

Update CMake requirements to v3.3.

Today's LLVM depends on CMake's CMP0057 (`if (IN_LIST ...)` operator)
working the new way, so let's update to a version of CMake where that's
true.