[SourceKit] Map line and column using the latest snapshot

During a cursor info request, the resolved offset was mapped to line and
column using the buffer inside `SwiftDocumentSyntaxInfo`.

However, prior to 54683ca,
`editorReplaceText` was not updating the syntax info. This meant that
snapshots would be more up to date than the buffer in the syntax info,
allowing for the possibility of an invalid offset.

While 54683ca would also fix this,
snapshots actually have a `getLineAndColumn` anyway. Use that instead of
grabbing the buffer from syntax info.

Resolves rdar://78161348.

Author: bnbarham

test/SourceKit/CursorInfo/cursor_after_edit.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+
+// RUN: touch %t/empty.swift
+// RUN: echo "func foo() {}" >> %t/func.swift
+
+// Edit previously did not update the syntax info. Cursor info was using its
+// buffer to calculate line and column (before rdar://78161348).
+// RUN: %sourcekitd-test \
+// RUN:   -req=open -text-input %t/empty.swift %t/func.swift -- %t/func.swift == \
+// RUN:   -req=edit -offset=0 -length=0 -replace="func foo() {}" -req-opts=enablesyntaxmap=0,enablesubstructure=0,enablediagnostics=0 %t/func.swift -- %t/func.swift == \
+// RUN:   -req=cursor -offset=5 %t/func.swift -- %t/func.swift

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -2308,17 +2308,6 @@ ImmutableTextSnapshotRef SwiftEditorDocument::getLatestSnapshot() const {
   return Impl.EditableBuffer->getSnapshot();
 }
 
-std::pair<unsigned, unsigned>
-SwiftEditorDocument::getLineAndColumnInBuffer(unsigned Offset) {
-  llvm::sys::ScopedLock L(Impl.AccessMtx);
-
-  auto SyntaxInfo = Impl.getSyntaxInfo();
-  auto &SM = SyntaxInfo->getSourceManager();
-
-  auto Loc = SM.getLocForOffset(SyntaxInfo->getBufferID(), Offset);
-  return SM.getLineAndColumnInBuffer(Loc);
-}
-
 void SwiftEditorDocument::reportDocumentStructure(SourceFile &SrcFile,
                                                   EditorConsumer &Consumer) {
   ide::SyntaxModelContext ModelContext(SrcFile);

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.h

@@ -105,7 +105,6 @@ class SwiftEditorDocument :
   void removeCachedAST();
 
   ImmutableTextSnapshotRef getLatestSnapshot() const;
-  std::pair<unsigned, unsigned> getLineAndColumnInBuffer(unsigned Offset);
 
   void resetSyntaxInfo(ImmutableTextSnapshotRef Snapshot,
                        SwiftLangSupport &Lang, bool BuildSyntaxTree,

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -639,7 +639,7 @@ static void mapLocToLatestSnapshot(
   }
 
   std::tie(Location.Line, Location.Column) =
-      EditorDoc->getLineAndColumnInBuffer(Location.Offset);
+      LatestSnap->getBuffer()->getLineAndColumn(Location.Offset);
 }