[clang][modules] NFCI: Avoid unnecessary serialization logic for non-affecting files

[jansvoboda11]

Summary

This patch delays some ASTWriter logic until after we've checked whether the source location entry we're serializing as an affecting file or not.

Depends on D137214.

[dexonsmith]

LGTM.

clang/lib/Serialization/ASTWriter.cpp

for (unsigned I = 1, N = SourceMgr.local_sloc_entry_size();
     // Get this source location entry.
     const SrcMgr::SLocEntry *SLoc = &SourceMgr.getLocalSLocEntry(I);
     FileID FID = FileID::get(I);
     assert(&SourceMgr.getSLocEntry(FID) == SLoc);
 
     // Record the offset of this source-location entry.
     uint64_t Offset = Stream.GetCurrentBitNo() - SLocEntryOffsetsBase;
     assert((Offset >> 32) == 0 && "SLocEntry offset too large");
-    SLocEntryOffsets.push_back(Offset);
 
     // Figure out which record code to use.
     unsigned Code;
     if (SLoc->isFile()) {
       const SrcMgr::ContentCache *Cache = &SLoc->getFile().getContentCache();
       if (Cache->OrigEntry) {
         Code = SM_SLOC_FILE_ENTRY;
       } else
         Code = SM_SLOC_BUFFER_ENTRY;
     } else
       Code = SM_SLOC_EXPANSION_ENTRY;
     Record.clear();
     Record.push_back(Code);
 
-    // Starting offset of this entry within this module, so skip the dummy.
-    Record.push_back(getAdjustedOffset(SLoc->getOffset()) - 2);
     if (SLoc->isFile()) {
       const SrcMgr::FileInfo &File = SLoc->getFile();
       const SrcMgr::ContentCache *Content = &File.getContentCache();
       if (Content->OrigEntry && !SkippedModuleMaps.empty() &&
           SkippedModuleMaps.find(Content->OrigEntry) !=
               SkippedModuleMaps.end()) {
         // Do not emit files that were not listed as inputs.
         continue;
       }
+      SLocEntryOffsets.push_back(Offset);
+      // Starting offset of this entry within this module, so skip the dummy.
+      Record.push_back(getAdjustedOffset(SLoc->getOffset()) - 2);
       AddSourceLocation(File.getIncludeLoc(), Record);
       Record.push_back(File.getFileCharacteristic()); // FIXME: stable encoding
       Record.push_back(File.hasLineDirectives());
 
       bool EmitBlob = false;
       if (Content->OrigEntry) {
         assert(Content->OrigEntry == Content->ContentsEntry &&
                "Writing to AST an overridden file is not supported");
if (SLoc->isFile()) {
           Buffer = llvm::MemoryBufferRef("<<<INVALID BUFFER>>>", "");
         StringRef Blob(Buffer->getBufferStart(), Buffer->getBufferSize() + 1);
         emitBlob(Stream, Blob, SLocBufferBlobCompressedAbbrv,
                  SLocBufferBlobAbbrv);
       }
     } else {
       // The source location entry is a macro expansion.
       const SrcMgr::ExpansionInfo &Expansion = SLoc->getExpansion();
+      SLocEntryOffsets.push_back(Offset);
+      // Starting offset of this entry within this module, so skip the dummy.
+      Record.push_back(getAdjustedOffset(SLoc->getOffset()) - 2);
       LocSeq::State Seq;
       AddSourceLocation(Expansion.getSpellingLoc(), Record, Seq);
       AddSourceLocation(Expansion.getExpansionLocStart(), Record, Seq);
       AddSourceLocation(Expansion.isMacroArgExpansion()
                             ? SourceLocation()
                             : Expansion.getExpansionLocEnd(),
                         Record, Seq);
       Record.push_back(Expansion.isExpansionTokenRange());