Include flags with relative directories always resolve relative to workspace

Problem

If a SourceFileConfiguration includes a entry in compilerFragments like /I..\foo, cpptools will attempt to resolve that include directory relative to the VS Code workspace root and silently discard the flag if the directory does not exist. However, it's possible that the compiler process was invoked with a different working directory than the workspace root, and the relative include directory would have resolved successfully relative to the compiler's working directory.

Suggested fix

Add a new readonly directory?: string; property to SourceFileConfiguration that specifies the working directory of the compiler process. Relative paths in compilerFragments should resolve relative to directory, if it is defined.

首先定位 SourceFileConfiguration 声明，以及将 compilerFragments 相对于 VS Code 工作区根目录进行解析的代码。确认编译器工作目录的表示方式，然后确保使用已定义的目录来处理相对 include 路径，并确保新的可选属性在公共 API 中公开。