[Serialization] Delta encode locations in expansion sloc entries

This is a 1.9% reduction in PCH size in my measurements.

In abbreviated records, VBR6 seems to be slightl better than VBR8 for locations
that may be delta-encoded (i.e. not the first)

Differential Revision: https://reviews.llvm.org/D125952

clang/lib/Serialization/ASTReader.cpp

@@ -1589,14 +1589,13 @@ bool ASTReader::ReadSLocEntry(int ID) {
   }
 
   case SM_SLOC_EXPANSION_ENTRY: {
-    SourceLocation SpellingLoc = ReadSourceLocation(*F, Record[1]);
-    SourceMgr.createExpansionLoc(SpellingLoc,
-                                     ReadSourceLocation(*F, Record[2]),
-                                     ReadSourceLocation(*F, Record[3]),
-                                     Record[5],
-                                     Record[4],
-                                     ID,
-                                     BaseOffset + Record[0]);
+    LocSeq::State Seq;
+    SourceLocation SpellingLoc = ReadSourceLocation(*F, Record[1], Seq);
+    SourceLocation ExpansionBegin = ReadSourceLocation(*F, Record[2], Seq);
+    SourceLocation ExpansionEnd = ReadSourceLocation(*F, Record[3], Seq);
+    SourceMgr.createExpansionLoc(SpellingLoc, ExpansionBegin, ExpansionEnd,
+                                 Record[5], Record[4], ID,
+                                 BaseOffset + Record[0]);
     break;
   }
   }

clang/lib/Serialization/ASTWriter.cpp

@@ -1715,8 +1715,8 @@ static unsigned CreateSLocExpansionAbbrev(llvm::BitstreamWriter &Stream) {
   Abbrev->Add(BitCodeAbbrevOp(SM_SLOC_EXPANSION_ENTRY));
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8)); // Offset
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8)); // Spelling location
-  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8)); // Start location
-  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8)); // End location
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // Start location
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // End location
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // Is token range
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // Token length
   return Stream.EmitAbbrev(std::move(Abbrev));
@@ -2139,12 +2139,13 @@ void ASTWriter::WriteSourceManagerBlock(SourceManager &SourceMgr,
     } else {
       // The source location entry is a macro expansion.
       const SrcMgr::ExpansionInfo &Expansion = SLoc->getExpansion();
-      AddSourceLocation(Expansion.getSpellingLoc(), Record);
-      AddSourceLocation(Expansion.getExpansionLocStart(), Record);
+      LocSeq::State Seq;
+      AddSourceLocation(Expansion.getSpellingLoc(), Record, Seq);
+      AddSourceLocation(Expansion.getExpansionLocStart(), Record, Seq);
       AddSourceLocation(Expansion.isMacroArgExpansion()
                             ? SourceLocation()
                             : Expansion.getExpansionLocEnd(),
-                        Record);
+                        Record, Seq);
       Record.push_back(Expansion.isExpansionTokenRange());
 
       // Compute the token length for this macro expansion.