fix(sema): remap imported builtin locations to import loc (#1305)

[0takuc0mrade]

Summary

Fixes #1305 by preventing diagnostic panics caused by Loc::Builtin notes when importing target builtins.

Problem

When builtin symbols are imported (for example from import "polkadot";) and then collide with user-defined symbols (for example chain_extension), diagnostics may include notes with Loc::Builtin.

Diagnostic rendering expects note locations to be file-backed (Loc::File) and could panic with:

internal error: entered unreachable code: note without file position

What Changed

• Added replace_builtin_loc(symbol, import_loc) in src/sema/mod.rs.
• In resolve_import, wrapped imported symbols with this helper before ns.add_symbol() for:
  • pt::Import::Rename
  • pt::Import::Plain
• replace_builtin_loc rewrites Loc::Builtin entries to the import statement location (import_loc) while leaving non-builtin locations unchanged.

Behavior Change

Before:

• Diagnostics could panic when a note referenced Loc::Builtin.

After:

• Diagnostics no longer panic.
• Notes for imported builtin symbols are anchored to the import statement location in the source file.

Scope / Safety

• Change is limited to import resolution (resolve_import).
• Only Loc::Builtin is remapped; all existing Loc::File locations are preserved.
• No semantic behavior changes outside diagnostic location mapping.

Validation

• Reproduced the collision scenario with imported builtin symbols.
• Confirmed panic is avoided and diagnostics are emitted normally with file-backed note locations.