[GR-73856] Ensure synthetic SourceRanges map to unavailable sections.

[graalvmbot]

This issue came up in an insight gate in the Graal repo. When parsed with args (as it is when insight is enabled), Python augments the module AST as:

def execute(args):
  <module body>

execute

using SourceRange.ARTIFICIAL_RANGE for the injected nodes. These ranges do not map to any meaningful source section, so SourceSection creation can fail. This change maps SourceRange.ARTIFICIAL_RANGE to an unavailable section.

For the "statement" of a function declaration we create a special source range with the length of the function name (using startLineShiftColumn), which also needs special handling for ARTIFICIAL_RANGE.