ISOSDacInterface::GetMethodDescName use proper path separator in cross-dac scenarios

[max-charlamb]

Found this bug while verifying the cDAC on ARM.

The existing DIRECTORY_SEPARATOR_CHAR_W defined in the PAL depends on the HOST_* not the TARGET_*. In this case, the path stored on the module is a unix style path and the directory structure isn't stripped correctly.

[Copilot]

Pull Request Overview

This PR fixes an issue in ISOSDacInterface::GetMethodDescName where the wrong directory separator was used for cross-DAC scenarios, causing Unix-style paths to be parsed incorrectly on non-Unix hosts.

• Introduce a directorySeparatorChar based on the target platform instead of using the host’s DIRECTORY_SEPARATOR_CHAR_W.
• Replace direct comparison against DIRECTORY_SEPARATOR_CHAR_W with the new directorySeparatorChar variable.

Comments suppressed due to low confidence (1)

src/coreclr/debug/daccess/request.cpp:1500

• Consider adding or updating unit tests for cross-DAC path parsing on non-Windows targets to verify that forward-slash separators are handled correctly.

                    while ((pFile >= path) && (*pFile != directorySeparatorChar))

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