[SourceKit] Fix a crash that occurred when a document without an associated source file is reopened

If a file isn’t opened with a `key.sourcefile` argument, we don’t store a snapshot for it. This could cause a crash when trying to consult its snapshot to see whether an AST can be reused for cursor info.

Author: ahoppen

test/SourceKit/Misc/no-crash-reopen-without-compiler-args.swift

@@ -0,0 +1,51 @@
+// RUN: %empty-directory(%t)
+// RUN: %{python} %utils/split_file.py -o %t %s
+// RUN: %sourcekitd-test \
+// RUN: -json-request-path %t/1-open-without-sourcefile.json \
+// RUN: == -json-request-path %t/2-close.json \
+// RUN: == -json-request-path %t/3-reopen-without-compiler-args.json \
+// RUN: == -json-request-path %t/4-cursor-info.json
+
+// This used to crash with a nullptr dereference because we didn't store a 
+// snapshot in the FileContents of a.swift since it wasn't opened with a 
+// key.sourcefile argument.
+
+// BEGIN 1-open-without-sourcefile.json
+{
+  key.request: source.request.editor.open,
+  key.name: "/invalid/a.swift",
+  key.compilerargs: [
+    "/invalid/a.swift",
+    "/invalid/b.swift"
+  ],
+  key.sourcetext: "",
+  key.enablesyntaxmap: 0,
+  key.enablesubstructure: 0,
+  key.enablediagnostics: 0
+}
+// BEGIN 2-close.json
+{
+  key.request: source.request.editor.close,
+  key.name: "/invalid/a.swift"
+}
+// BEGIN 3-reopen-without-compiler-args.json
+{
+  key.request: source.request.editor.open,
+  key.name: "/invalid/a.swift",
+  key.compilerargs: [
+  ],
+  key.sourcetext: "",
+  key.enablesyntaxmap: 0,
+  key.enablesubstructure: 0,
+  key.enablediagnostics: 0
+}
+// BEGIN 4-cursor-info.json
+{
+  key.request: source.request.cursorinfo,
+  key.compilerargs: [
+    "/invalid/a.swift",
+    "/invalid/b.swift"
+  ],
+  key.offset: 0,
+  key.sourcefile: "/invalid/a.swift"
+}

tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp

@@ -1185,7 +1185,9 @@ ASTBuildOperationRef ASTProducer::getBuildOperationForConsumer(
     std::vector<ImmutableTextSnapshotRef> Snapshots;
     Snapshots.reserve(BuildOp->getFileContents().size());
     for (auto &FileContent : BuildOp->getFileContents()) {
-      Snapshots.push_back(FileContent.Snapshot);
+      if (FileContent.Snapshot) {
+        Snapshots.push_back(FileContent.Snapshot);
+      }
     }
     if (BuildOp->matchesSourceState(FileSystem)) {
       ++Mgr->Impl.Stats->numASTCacheHits;