[clang][Tooling] Get rid of a hack in SymbolOccurrences, NFCI

The class `SymbolOccurrences` can store either a single `SourceRange`
in-place or multiple `SourceRanges` on the heap. In the latter case
the number of source ranges is stored in the internal representation
of the beginning `SourceLocation` of the in-place `SourceRange`
object.

This change gets rid of such hack by placing `SourceRange` in a union
which holds either a valid `SourceRange` or an `unsigned int` (a number
of ranges).

The change also adds `static_assert`s that check that `SourceRange` and
`SourceLocation` are trivially destructible (this is required for the
current patch and for D94237 which has already been committed).

Reviewed By: MarkMurrayARM, simon_tatham

Differential Revision: https://reviews.llvm.org/D94599

Author: miyuki

clang/include/clang/Tooling/Refactoring/Rename/SymbolOccurrences.h

@@ -69,17 +69,18 @@ class SymbolOccurrence {
   OccurrenceKind getKind() const { return Kind; }
 
   ArrayRef<SourceRange> getNameRanges() const {
-    if (MultipleRanges) {
-      return llvm::makeArrayRef(MultipleRanges.get(),
-                                RangeOrNumRanges.getBegin().getRawEncoding());
-    }
-    return RangeOrNumRanges;
+    if (MultipleRanges)
+      return llvm::makeArrayRef(MultipleRanges.get(), NumRanges);
+    return SingleRange;
   }
 
 private:
   OccurrenceKind Kind;
   std::unique_ptr<SourceRange[]> MultipleRanges;
-  SourceRange RangeOrNumRanges;
+  union {
+    SourceRange SingleRange;
+    unsigned NumRanges;
+  };
 };
 
 using SymbolOccurrences = std::vector<SymbolOccurrence>;

clang/lib/Basic/SourceLocation.cpp

@@ -42,6 +42,14 @@ void PrettyStackTraceLoc::print(raw_ostream &OS) const {
 // SourceLocation
 //===----------------------------------------------------------------------===//
 
+static_assert(std::is_trivially_destructible<SourceLocation>::value,
+              "SourceLocation must be trivially destructible because it is "
+              "used in unions");
+
+static_assert(std::is_trivially_destructible<SourceRange>::value,
+              "SourceRange must be trivially destructible because it is "
+              "used in unions");
+
 unsigned SourceLocation::getHashValue() const {
   return llvm::DenseMapInfo<unsigned>::getHashValue(ID);
 }

clang/lib/Tooling/Refactoring/Rename/SymbolOccurrences.cpp

@@ -21,13 +21,12 @@ SymbolOccurrence::SymbolOccurrence(const SymbolName &Name, OccurrenceKind Kind,
          "mismatching number of locations and lengths");
   assert(!Locations.empty() && "no locations");
   if (Locations.size() == 1) {
-    RangeOrNumRanges = SourceRange(
+    new (&SingleRange) SourceRange(
         Locations[0], Locations[0].getLocWithOffset(NamePieces[0].size()));
     return;
   }
   MultipleRanges = std::make_unique<SourceRange[]>(Locations.size());
-  RangeOrNumRanges.setBegin(
-      SourceLocation::getFromRawEncoding(Locations.size()));
+  NumRanges = Locations.size();
   for (const auto &Loc : llvm::enumerate(Locations)) {
     MultipleRanges[Loc.index()] = SourceRange(
         Loc.value(),