[clang] [clang][modules] Remove preloaded SLocEntries from PCM files (PR #66962)

[Ben Langmuir via cfe-commits]

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@@ -701,6 +701,10 @@ class SourceManager : public RefCountedBase<SourceManager> {
   /// use (-ID - 2).
   SmallVector<SrcMgr::SLocEntry, 0> LoadedSLocEntryTable;
 
+  /// For each allocation in LoadedSLocEntryTable, we keep the new size. This
+  /// can be used to determine whether two FileIDs come from the same AST file.
+  SmallVector<size_t, 0> LoadedSLocEntryTableSegments;
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benlangmuir wrote:

> I guess I could store `FileID(ModuleFile::SLocEntryBaseID - 1)` instead (which is the module's `FileID(1)` translated into the importer address space for loaded entries) and do `lower_bound` over that:
> 
> ```c++
> SmallVector<FileID, 0> LoadedSLocEntryTableSegments{
>   FileID(-10),  // represents FileIDs from -10 to -2
>   FileID(-50),  // represents FileIDs from -50 to -11
>   FileID(-90)}; // represents FileIDs from -90 to -51
> 
> llvm::lower_bound(LoadedSLocEntryTableSegments, FID, [](FileID Element, FileID Value) {
>   return Element.ID > Value.ID;
> });
> ```
> 
> The lambda is still confusing... Any better ideas?

This is roughly what I had in mind, though I would just do `std::greater` for that comparison function (FileID has comparison operators) and leave a comment that loaded FileID is negative.  I find this much clearer than searching for `-ID - 2`.

https://github.com/llvm/llvm-project/pull/66962