Relative problem paths don't take `cwd` into account

From https://github.com/Microsoft/vscode-cpptools/issues/2518 by @galgalesh -------------------------------------------------------- **Type: LanguageService** <!----- Input information below -----> <!-- **Please review existing issues and our documentation at https://github.com/Microsoft/vscode-cpptools/tree/master/Documentation prior to filing an issue.** --> **Describe the bug** - OS and Version: Ubuntu 18.04 - VS Code Version: 1.27.1 5944e81f3c46a3938a82c701f96d7a59b074cfdc x64 - C/C++ Extension Version: 0.18.1 - Other extensions you installed (and if the issue persists after disabling them): - A clear and concise description of what the bug is. The problemMatcher doesn't take the `cwd` of a task into account when resolving relative paths. **To Reproduce** <!-- Steps to reproduce the behavior: --> <!-- *The most actionable issue reports include a code sample including configuration files such as c_cpp_properties.json* --> Create a task and set the `cwd` of that task to a different path. **Expected behavior** The problemMatcher resolves relative paths as relative to `cwd`. If you set the `cwd` of a task, relative paths will most likely be relative to the `cwd`, not to the workspaceFolder. **Behavior I get** The problemMatcher resolves relative paths as relative to `workspaceFolder` Following is the `tasks.json` for such a task. The line starting with ` "fileLocation"` shouldn't be neccisary. ``` { "label": "build adfgxx", "type": "shell", "command": "ninja", "args": [ ], "group": { "kind": "build", "isDefault": true }, "problemMatcher": { "base": "$gcc", // The next line shouldn't be needed since it is the same value as `cwd` "fileLocation" : ["relative", "${workspaceFolder}/builddir"] }, "options": { "cwd": "${workspaceFolder}/builddir", …