[driver] Add -working-directory option

Adds a -working-directory option which can be used to modify how
relative paths are resolved. It affects all other paths used in driver
options (controlled by a new ArgumentIsPath flag on options) as well as
the contents of output file maps and auxilliary file paths generated
implicitly by the compiler itself.

rdar://37713856

include/swift/Driver/OutputFileMap.h

@@ -39,14 +39,23 @@ public:
   ~OutputFileMap() = default;
 
   /// Loads an OutputFileMap from the given \p Path, if possible.
-  static std::unique_ptr<OutputFileMap> loadFromPath(StringRef Path);
+  ///
+  /// When non-empty, \p workingDirectory is used to resolve relative paths in
+  /// the output file map.
+  static std::unique_ptr<OutputFileMap>
+  loadFromPath(StringRef Path, StringRef workingDirectory);
 
-  static std::unique_ptr<OutputFileMap> loadFromBuffer(StringRef Data);
+  static std::unique_ptr<OutputFileMap>
+  loadFromBuffer(StringRef Data, StringRef workingDirectory);
 
   /// Loads an OutputFileMap from the given \p Buffer, taking ownership
   /// of the buffer in the process.
+  ///
+  /// When non-empty, \p workingDirectory is used to resolve relative paths in
+  /// the output file map.
   static std::unique_ptr<OutputFileMap>
-  loadFromBuffer(std::unique_ptr<llvm::MemoryBuffer> Buffer);
+  loadFromBuffer(std::unique_ptr<llvm::MemoryBuffer> Buffer,
+                 StringRef workingDirectory);
 
   /// Get the map of outputs for the given \p Input, if present in the
   /// OutputFileMap. (If not present, returns nullptr.)
@@ -69,8 +78,12 @@ private:
   /// \brief Parses the given \p Buffer into the OutputFileMap, taking ownership
   /// of \p Buffer in the process.
   ///
+  /// When non-empty, \p workingDirectory is used to resolve relative paths in
+  /// the output file map.
+  ///
   /// \returns true on error, false on success
-  bool parse(std::unique_ptr<llvm::MemoryBuffer> Buffer);
+  bool parse(std::unique_ptr<llvm::MemoryBuffer> Buffer,
+             StringRef workingDirectory);
 };
 
 } // end namespace driver