[clang][modules] NFCI: Pragma diagnostic mappings: write/read FileID instead of SourceLocation (llvm#87427)

For pragma diagnostic mappings, we always write/read `SourceLocation`
with offset 0. This is equivalent to just writing a `FileID`, which is
exactly what this patch starts doing.

Originally reviewed here: https://reviews.llvm.org/D137213

(cherry picked from commit c925c16)

Author: jansvoboda11

clang/include/clang/Serialization/ASTBitCodes.h

@@ -41,7 +41,7 @@ namespace serialization {
 /// Version 4 of AST files also requires that the version control branch and
 /// revision match exactly, since there is no backward compatibility of
 /// AST files at this time.
-const unsigned VERSION_MAJOR = 29;
+const unsigned VERSION_MAJOR = 30;
 
 /// AST file minor version number supported by this version of
 /// Clang.

clang/lib/Serialization/ASTReader.cpp

@@ -6604,17 +6604,17 @@ void ASTReader::ReadPragmaDiagnosticMappings(DiagnosticsEngine &Diag) {
     while (NumLocations--) {
       assert(Idx < Record.size() &&
              "Invalid data, missing pragma diagnostic states");
-      SourceLocation Loc = ReadSourceLocation(F, Record[Idx++]);
-      auto IDAndOffset = SourceMgr.getDecomposedLoc(Loc);
-      assert(IDAndOffset.first.isValid() && "invalid FileID for transition");
-      assert(IDAndOffset.second == 0 && "not a start location for a FileID");
+      FileID FID = ReadFileID(F, Record, Idx);
+      assert(FID.isValid() && "invalid FileID for transition");
+      // FIXME: Remove this once we don't need the side-effects.
+      (void)SourceMgr.getSLocEntryOrNull(FID);
       unsigned Transitions = Record[Idx++];
 
       // Note that we don't need to set up Parent/ParentOffset here, because
       // we won't be changing the diagnostic state within imported FileIDs
       // (other than perhaps appending to the main source file, which has no
       // parent).
-      auto &F = Diag.DiagStatesByLoc.Files[IDAndOffset.first];
+      auto &F = Diag.DiagStatesByLoc.Files[FID];
       F.StateTransitions.reserve(F.StateTransitions.size() + Transitions);
       for (unsigned I = 0; I != Transitions; ++I) {
         unsigned Offset = Record[Idx++];

clang/lib/Serialization/ASTWriter.cpp

@@ -3131,9 +3131,7 @@ void ASTWriter::WritePragmaDiagnosticMappings(const DiagnosticsEngine &Diag,
       continue;
     ++NumLocations;
 
-    SourceLocation Loc = Diag.SourceMgr->getComposedLoc(FileIDAndFile.first, 0);
-    assert(!Loc.isInvalid() && "start loc for valid FileID is invalid");
-    AddSourceLocation(Loc, Record);
+    AddFileID(FileIDAndFile.first, Record);
 
     Record.push_back(FileIDAndFile.second.StateTransitions.size());
     for (auto &StatePoint : FileIDAndFile.second.StateTransitions) {