fix: escape response-file backslashes for driver linkers

[ghaith]

Problem

On Windows, linking a real-sized project through the clang driver fails with no such file or directory for every object:

clang: error: no such file or directory: 'C:bachmannSolutionCenterworkspace...mainProg.st.o'

Every backslash has been stripped: C:\build\obj\mainProg.st.o arrives as C:buildobjmainProg.st.o.

Cause

write_response_file assumed a single tokenizer for all linkers. Its comment stated "an unquoted backslash is literal in this format" — true for lld and binutils, but a compiler driver treats a backslash as an escape. Object paths were written bare, so the driver consumed the separators.

The defect needed two conditions that could not co-occur on Windows until recently:

1. A command line over the 30,000-byte threshold, routing args through a response file. Small projects never reach it.
2. A driver linker consuming that file. Windows always fell back to a direct ld.lld link, which reads backslashes literally — so the existing behaviour was correct for the only linker that could be selected.

Including the sysroot in the driver probe (#1852) made clang selectable on Windows and switched on condition 2, exposing the writer's assumption. The bug was latent in the writer the whole time; nothing on Windows could reach it.

Fix

Choose the escaping from the linker family where the file is written:

Linker family	Style	Backslash
clang, cc, gcc	EscapedBackslash	doubled
ld.lld, ld.bfd, gold	LiteralBackslash	literal

Quoted arguments (those carrying whitespace) already double backslashes under both families, so that path is unchanged.

Verification

Forcing a response file with PLC_LINKER_RESPONSE_FILE=1 on a Windows host:

Linker	Before	After
clang (driver)	fails on every object	links
ld.lld (direct)	links	links

Tests

30 linker unit tests pass, three of them new and each failing without this change:

• response_file_quoting_escapes_backslashes_for_driver_linkers
• write_response_file_escapes_object_paths_for_driver_linkers — asserts the emitted lines and that each unescapes back to the original argument
• driver_linker_response_file_round_trips_windows_paths — goes through CcLinker so the style is derived the way finalize derives it, rather than asserted against a hand-passed constant

No lit test. A lit case could force a response file with a driver linker, but lit runs on Linux where paths carry no backslashes, so the defect cannot reproduce — it would pass for the wrong reason. The unit tests are the real guard.

🤖 Generated with Claude Code

[github-actions]

Build Artifacts

🐧 Linux

Artifact	Link	Size
deb-x86_64	Download	38.4 MB
schema	Download	0.0 MB
stdlib	Download	32.4 MB
plc-x86_64	Download	43.5 MB
deb-aarch64	Download	30.8 MB
plc-aarch64	Download	43.3 MB

From workflow run

🪟 Windows

Artifact	Link	Size
stdlib.lib	Download	4.0 MB
stdlib.dll	Download	0.1 MB
plc.exe	Download	38.3 MB

From workflow run