[Modules] Change result of reading AST block to llvm::Error instead

Reading the AST block can never fail with a recoverable error as modules
cannot be removed during this phase. Change the return type of these
functions to return an llvm::Error instead, ie. either success or
failure.

NFC other than the wording of some of the errors.

Differential Revision: https://reviews.llvm.org/D108268

Author: bnbarham

clang/include/clang/Serialization/ASTReader.h

@@ -1313,18 +1313,18 @@ class ASTReader
                                ASTReaderListener *Listener,
                                bool ValidateDiagnosticOptions);
 
-  ASTReadResult ReadASTBlock(ModuleFile &F, unsigned ClientLoadCapabilities);
-  ASTReadResult ReadExtensionBlock(ModuleFile &F);
+  llvm::Error ReadASTBlock(ModuleFile &F, unsigned ClientLoadCapabilities);
+  llvm::Error ReadExtensionBlock(ModuleFile &F);
   void ReadModuleOffsetMap(ModuleFile &F) const;
-  bool ParseLineTable(ModuleFile &F, const RecordData &Record);
-  bool ReadSourceManagerBlock(ModuleFile &F);
+  void ParseLineTable(ModuleFile &F, const RecordData &Record);
+  llvm::Error ReadSourceManagerBlock(ModuleFile &F);
   llvm::BitstreamCursor &SLocCursorForID(int ID);
   SourceLocation getImportLocation(ModuleFile *F);
   ASTReadResult ReadModuleMapFileBlock(RecordData &Record, ModuleFile &F,
                                        const ModuleFile *ImportedBy,
                                        unsigned ClientLoadCapabilities);
-  ASTReadResult ReadSubmoduleBlock(ModuleFile &F,
-                                   unsigned ClientLoadCapabilities);
+  llvm::Error ReadSubmoduleBlock(ModuleFile &F,
+                                 unsigned ClientLoadCapabilities);
   static bool ParseLanguageOptions(const RecordData &Record, bool Complain,
                                    ASTReaderListener &Listener,
                                    bool AllowCompatibleDifferences);
@@ -1896,8 +1896,9 @@ class ASTReader
   /// ReadBlockAbbrevs - Enter a subblock of the specified BlockID with the
   /// specified cursor.  Read the abbreviations that are at the top of the block
   /// and then leave the cursor pointing into the block.
-  static bool ReadBlockAbbrevs(llvm::BitstreamCursor &Cursor, unsigned BlockID,
-                               uint64_t *StartOfBlockOffset = nullptr);
+  static llvm::Error ReadBlockAbbrevs(llvm::BitstreamCursor &Cursor,
+                                      unsigned BlockID,
+                                      uint64_t *StartOfBlockOffset = nullptr);
 
   /// Finds all the visible declarations with a given name.
   /// The current implementation of this method just loads the entire