[mlir][BytecodeReader] Const qualify *SectionReader, NFC (#99376)

Summary:
`StringSectionReader`, `ResourceSectionReader` and
`PropertiesSectionReader` are immutable after `initialize` so this PR
adds const to their parsing functions and references in `AttrTypeReader`
and `DialectReader`.

Test Plan: 

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Tasks: 

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Differential Revision: https://phabricator.intern.facebook.com/D60250798

Author: uenoku

mlir/lib/Bytecode/Reader/BytecodeReader.cpp

@@ -375,15 +375,15 @@ class StringSectionReader {
 
   /// Parse a shared string from the string section. The shared string is
   /// encoded using an index to a corresponding string in the string section.
-  LogicalResult parseString(EncodingReader &reader, StringRef &result) {
+  LogicalResult parseString(EncodingReader &reader, StringRef &result) const {
     return parseEntry(reader, strings, result, "string");
   }
 
   /// Parse a shared string from the string section. The shared string is
   /// encoded using an index to a corresponding string in the string section.
   /// This variant parses a flag compressed with the index.
   LogicalResult parseStringWithFlag(EncodingReader &reader, StringRef &result,
-                                    bool &flag) {
+                                    bool &flag) const {
     uint64_t entryIdx;
     if (failed(reader.parseVarIntWithFlag(entryIdx, flag)))
       return failure();
@@ -393,7 +393,7 @@ class StringSectionReader {
   /// Parse a shared string from the string section. The shared string is
   /// encoded using an index to a corresponding string in the string section.
   LogicalResult parseStringAtIndex(EncodingReader &reader, uint64_t index,
-                                   StringRef &result) {
+                                   StringRef &result) const {
     return resolveEntry(reader, strings, index, result, "string");
   }
 
@@ -544,7 +544,7 @@ class ResourceSectionReader {
 
   /// Parse a dialect resource handle from the resource section.
   LogicalResult parseResourceHandle(EncodingReader &reader,
-                                    AsmDialectResourceHandle &result) {
+                                    AsmDialectResourceHandle &result) const {
     return parseEntry(reader, dialectResources, result, "resource handle");
   }
 
@@ -800,8 +800,8 @@ class AttrTypeReader {
   using TypeEntry = Entry<Type>;
 
 public:
-  AttrTypeReader(StringSectionReader &stringReader,
-                 ResourceSectionReader &resourceReader,
+  AttrTypeReader(const StringSectionReader &stringReader,
+                 const ResourceSectionReader &resourceReader,
                  const llvm::StringMap<BytecodeDialect *> &dialectsMap,
                  uint64_t &bytecodeVersion, Location fileLoc,
                  const ParserConfig &config)
@@ -881,11 +881,11 @@ class AttrTypeReader {
 
   /// The string section reader used to resolve string references when parsing
   /// custom encoded attribute/type entries.
-  StringSectionReader &stringReader;
+  const StringSectionReader &stringReader;
 
   /// The resource section reader used to resolve resource references when
   /// parsing custom encoded attribute/type entries.
-  ResourceSectionReader &resourceReader;
+  const ResourceSectionReader &resourceReader;
 
   /// The map of the loaded dialects used to retrieve dialect information, such
   /// as the dialect version.
@@ -908,8 +908,8 @@ class AttrTypeReader {
 class DialectReader : public DialectBytecodeReader {
 public:
   DialectReader(AttrTypeReader &attrTypeReader,
-                StringSectionReader &stringReader,
-                ResourceSectionReader &resourceReader,
+                const StringSectionReader &stringReader,
+                const ResourceSectionReader &resourceReader,
                 const llvm::StringMap<BytecodeDialect *> &dialectsMap,
                 EncodingReader &reader, uint64_t &bytecodeVersion)
       : attrTypeReader(attrTypeReader), stringReader(stringReader),
@@ -1042,8 +1042,8 @@ class DialectReader : public DialectBytecodeReader {
 
 private:
   AttrTypeReader &attrTypeReader;
-  StringSectionReader &stringReader;
-  ResourceSectionReader &resourceReader;
+  const StringSectionReader &stringReader;
+  const ResourceSectionReader &resourceReader;
   const llvm::StringMap<BytecodeDialect *> &dialectsMap;
   EncodingReader &reader;
   uint64_t &bytecodeVersion;
@@ -1082,7 +1082,7 @@ class PropertiesSectionReader {
   }
 
   LogicalResult read(Location fileLoc, DialectReader &dialectReader,
-                     OperationName *opName, OperationState &opState) {
+                     OperationName *opName, OperationState &opState) const {
     uint64_t propertiesIdx;
     if (failed(dialectReader.readVarInt(propertiesIdx)))
       return failure();