Do not include the full file path in check assertion output

[vchlin]

The current output of a check assertion includes std::source_location::file_name:

corofx/src/check.cpp

Lines 14 to 18 in 8e0d439

	auto check(bool pred, std::source_location loc) noexcept -> void {
	if (pred) return;
	trace(loc.file_name(), ":", loc.line(), ": ", loc.function_name(), ": check failed");
	std::terminate();
	}

In certain implementations, this can expose the absolute path of the file being compiled, potentially revealing the directory structure of the developer's environment.

A better approach might be to keep only the file's base name or remove the path components leading up to the project's directory structure.