[lldb][NFCI] Remove use of ConstString in ProcessStructReader

[bulbazord]

Summary

std::map<ConstString, FieldImpl> is naturally replaced with
llvm::StringMap<FieldImpl> here.

lldb/include/lldb/Target/ProcessStructReader.h

 //===---------------------ProcessStructReader.h ------------------*- C++-*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLDB_TARGET_PROCESSSTRUCTREADER_H
 #define LLDB_TARGET_PROCESSSTRUCTREADER_H
 
 #include "lldb/lldb-defines.h"
 #include "lldb/lldb-types.h"
 
 #include "lldb/Symbol/CompilerType.h"
 #include "lldb/Target/Process.h"
-#include "lldb/Utility/ConstString.h"
 #include "lldb/Utility/DataBufferHeap.h"
 #include "lldb/Utility/DataExtractor.h"
 #include "lldb/Utility/Status.h"
 
+#include "llvm/ADT/StringMap.h"
+
 #include <initializer_list>
 #include <map>
 #include <string>
 
 namespace lldb_private {
 class ProcessStructReader {
 protected:
   struct FieldImpl {
     CompilerType type;
     size_t offset;
     size_t size;
   };
 
-  std::map<ConstString, FieldImpl> m_fields;
+  llvm::StringMap<FieldImpl> m_fields;
   DataExtractor m_data;
   lldb::ByteOrder m_byte_order;
   size_t m_addr_byte_size;
 
 public:
   ProcessStructReader(Process *process, lldb::addr_t base_addr,
                       CompilerType struct_type)
       : m_byte_order(lldb::eByteOrderInvalid), m_addr_byte_size(0) {
for (size_t idx = 0; idx < struct_type.GetNumFields(); idx++) {
           idx, name, &bit_offset, &bitfield_bit_size, &is_bitfield);
       // no support for bitfields in here (yet)
       if (is_bitfield)
         return;
       auto size = field_type.GetByteSize(nullptr);
       // no support for things larger than a uint64_t (yet)
       if (!size || *size > 8)
         return;
-      ConstString const_name = ConstString(name.c_str());
       size_t byte_index = static_cast<size_t>(bit_offset / 8);
-      m_fields[const_name] =
-          FieldImpl{field_type, byte_index, static_cast<size_t>(*size)};
+      m_fields.insert({name, FieldImpl{field_type, byte_index,
+                                       static_cast<size_t>(*size)}});
     }
     auto total_size = struct_type.GetByteSize(nullptr);
     if (!total_size)
       return;
     lldb::WritableDataBufferSP buffer_sp(new DataBufferHeap(*total_size, 0));
     Status error;
     process->ReadMemoryFromInferior(base_addr, buffer_sp->GetBytes(),
                                     *total_size, error);
     if (error.Fail())
       return;
     m_data = DataExtractor(buffer_sp, m_byte_order, m_addr_byte_size);
   }
 
   template <typename RetType>
-  RetType GetField(ConstString name, RetType fail_value = RetType()) {
+  RetType GetField(llvm::StringRef name, RetType fail_value = RetType()) {
     auto iter = m_fields.find(name), end = m_fields.end();
     if (iter == end)
       return fail_value;
     auto size = iter->second.size;
     if (sizeof(RetType) < size)
       return fail_value;
     lldb::offset_t offset = iter->second.offset;
     if (offset + size > m_data.GetByteSize())
       return fail_value;
     return (RetType)(m_data.GetMaxU64(&offset, size));
   }
 };
 }
 
 #endif // LLDB_TARGET_PROCESSSTRUCTREADER_H

lldb/source/Plugins/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp

if (m_dispatch_tsd_indexes_addr != LLDB_INVALID_ADDRESS) {
                                             "dti_qos_class_index", uint16,
                                             lldb::eAccessPublic, 0);
       TypeSystemClang::CompleteTagDeclarationDefinition(dispatch_tsd_indexes_s);
 
       ProcessStructReader struct_reader(m_process, m_dispatch_tsd_indexes_addr,
                                         dispatch_tsd_indexes_s);
 
       m_libdispatch_tsd_indexes.dti_version =
-          struct_reader.GetField<uint16_t>(ConstString("dti_version"));
+          struct_reader.GetField<uint16_t>("dti_version");
       m_libdispatch_tsd_indexes.dti_queue_index =
-          struct_reader.GetField<uint16_t>(ConstString("dti_queue_index"));
+          struct_reader.GetField<uint16_t>("dti_queue_index");
       m_libdispatch_tsd_indexes.dti_voucher_index =
-          struct_reader.GetField<uint16_t>(ConstString("dti_voucher_index"));
+          struct_reader.GetField<uint16_t>("dti_voucher_index");
       m_libdispatch_tsd_indexes.dti_qos_class_index =
-          struct_reader.GetField<uint16_t>(ConstString("dti_qos_class_index"));
+          struct_reader.GetField<uint16_t>("dti_qos_class_index");
     }
   }
 }
 
 ThreadSP SystemRuntimeMacOSX::GetExtendedBacktraceThread(ThreadSP real_thread,
                                                          ConstString type) {
   ThreadSP originating_thread_sp;
   if (BacktraceRecordingHeadersInitialized() && type == "libdispatch") {