fix(sema): keep builtin import notes on source locations

[malsomesh9]

Summary

Fix builtin import diagnostics so duplicate-definition notes point to source locations instead of crashing when the original definition comes from a builtin.

Changes

• rewrite imported builtin function note locations to use the import site
• report builtin duplicate definitions as already defined as a builtin function
• make the diagnostic renderer tolerate any remaining notes without file locations
• add a regression test for importing polkadot and redefining chain_extension

Verification

• cargo fmt --all
• attempted targeted regression test, but this machine does not have a system LLVM installation for llvm-sys, which blocks the default Solang test build

Fixes #1308