Added hack to prevent toHalfOpenFileRange assertion fail

[SureYeaah]

Summary

• Still need to investigate why this is happening.
• Fixes the crash described at https://github.com/clangd/clangd/issues/113

[sammccall]

clang-tools-extra/clangd/Selection.cpp

373

This isn't a sufficient fix, there are 5 callsites that don't check for failure.
Adding defenses to each of them is the right thing if we really can't handle failure, but today they assume the invariant that if the inputs were from the same file id, the output will be valid. Can we really not provide that guarantee?

clang-tools-extra/clangd/SourceCode.cpp

314

We have a reproducer, why can't we investigate now?

[SureYeaah]

clang-tools-extra/clangd/SourceCode.cpp

314

Doing that. Just wanted to stop the crashes in the meanwhile.

clang-tools-extra/clangd/Selection.cpp

if (!S.isValid())
       return SelectionTree::Unselected;
     // toHalfOpenFileRange() allows selection of constructs in macro args. e.g:
     //   #define LOOP_FOREVER(Body) for(;;) { Body }
     //   void IncrementLots(int &x) {
     //     LOOP_FOREVER( ++x; )
     //   }
     // Selecting "++x" or "x" will do the right thing.
     auto Range = toHalfOpenFileRange(SM, LangOpts, S);
-    assert(Range && "We should be able to get the File Range");
+    if(!Range)
+      return SelectionTree::Unselected;
     dlog("{1}claimRange: {0}", Range->printToString(SM), indent());
     auto B = SM.getDecomposedLoc(Range->getBegin());
     auto E = SM.getDecomposedLoc(Range->getEnd());
     // Otherwise, nodes in macro expansions can't be selected.
     if (B.first != SelFile || E.first != SelFile)
       return SelectionTree::Unselected;
     // Attempt to claim the remaining range. If there's nothing to claim, only
     // children were selected.

clang-tools-extra/clangd/SourceCode.cpp

 //   instantiations as a '>>' instead of a '>'.
 // There is also getExpansionRange but it simply calls
 // getImmediateExpansionRange on the begin and ends separately which is wrong.
 static SourceRange getTokenFileRange(SourceLocation Loc,
                                      const SourceManager &SM,
                                      const LangOptions &LangOpts) {
   SourceRange FileRange = Loc;
   while (!FileRange.getBegin().isFileID()) {
-    assert(!FileRange.getEnd().isFileID() &&
-           "Both Begin and End should be MacroIDs.");
+    // FIXME: Investigate when this assert fails. Added a hack until then.
+    // assert(!FileRange.getEnd().isFileID() &&
+    //       "Both Begin and End should be MacroIDs.");
+    if(FileRange.getEnd().isFileID())
+      return SourceRange();
     if (SM.isMacroArgExpansion(FileRange.getBegin())) {
       FileRange.setBegin(SM.getImmediateSpellingLoc(FileRange.getBegin()));
       FileRange.setEnd(SM.getImmediateSpellingLoc(FileRange.getEnd()));
     } else {
       SourceRange ExpansionRangeForBegin = toTokenRange(
           SM.getImmediateExpansionRange(FileRange.getBegin()), SM, LangOpts);
       SourceRange ExpansionRangeForEnd = toTokenRange(
           SM.getImmediateExpansionRange(FileRange.getEnd()), SM, LangOpts);