[lldb/Host] Remove TaskPool and replace its uses with llvm::ThreadPool

[JDevlieghere]

Remove LLDB's TaskPool and replace its uses with LLVM's ThreadPool.

Differential revision: https://reviews.llvm.org/D78337

(cherry picked from commit e57361c)

[JDevlieghere]

@swift-ci please test

[varungandhi-apple]

Thank you! 😄

[JDevlieghere]

I don't see the REPL test timeouts locally and I can't think of any reason this change would cause that. Let's try again...

@swift-ci please test platform macos

[JDevlieghere]

@swift-ci please test macos platform

[JDevlieghere]

After a few tries I was able to repro this. It appears like a legit deadlock. We're waiting on the module mutex in ObjectFileMachO::ParseHeader().

frame #4: 0x000000011583308f LLDB`ObjectFileMachO::ParseHeader() 
frame #5: 0x0000000115833087 LLDB`ObjectFileMachO::ParseHeader() 
frame #6: 0x0000000115833087 LLDB`ObjectFileMachO::ParseHeader(...) at ObjectFileMachO.cpp:984 [opt]
frame #7: 0x00000001158312a7 LLDB`ObjectFileMachO::CreateInstance(...) at ObjectFileMachO.cpp:801:35 [opt]
frame #8: 0x000000011556ec0b LLDB`lldb_private::ObjectFile::FindPlugin(...) at ObjectFile.cpp:137:32 [opt]
frame #9: 0x0000000115959fff LLDB`SymbolFileDWARF::GetDwoSymbolFileForCompileUnit(...) at SymbolFileDWARF.cpp:1723:31 [opt]
frame #10: 0x000000011594618b LLDB`DWARFUnit::AddUnitDIE(...) at DWARFUnit.cpp:340:15 [opt]
frame #11: 0x00000001159469c1 LLDB`DWARFUnit::ExtractDIEsRWLocked(...) at DWARFUnit.cpp:184:9 [opt]
frame #12: 0x0000000115946db0 LLDB`DWARFUnit::ExtractDIEsScoped(...) at DWARFUnit.cpp:107:3 [opt]

I'll need to do a debug build to be sure, but I suspect it's the same module as the one held by the SymbolFileDWARF instance where we call SymbolFileDWARF::FindFunctions.

frame #5: 0x000000011594d9c2 LLDB`lldb_private::ManualDWARFIndex::Index(...) at ManualDWARFIndex.cpp:72:8 [opt]
frame #6: 0x000000011594eb90 LLDB`lldb_private::ManualDWARFIndex::GetFunctions(...), dwarf=0x00007fd8c6221e00, parent_decl_ctx=0x0000700001fb58c0, name_type_mask=4, dies=size=0) at ManualDWARFIndex.cpp:361:3 [opt]
frame #7: 0x000000011595cd24 LLDB`SymbolFileDWARF::FindFunctions(...) at SymbolFileDWARF.cpp:2449:12 [opt]
frame #8: 0x00000001154830dc LLDB`lldb_private::Module::FindFunctions(...), parent_decl_ctx=0x0000000000000000, name_type_mask=eFunctionNameTypeFull, include_symbols=true, include_inlines=false, sc_list=0x0000700001fb5af8) at Module.cpp:849:16 [opt]
frame #9: 0x0000000115488416 LLDB`lldb_private::ModuleList::FindFunctions(...) const at ModuleList.cpp:410:15 [opt]
frame #10: 0x00000001154e545c LLDB`lldb_private::IRExecutionUnit::FindInSymbols(...) at IRExecutionUnit.cpp:880:33 [opt]
frame #11: 0x00000001154e5781 LLDB`lldb_private::IRExecutionUnit::FindSymbol(...) at IRExecutionUnit.cpp:960:22 [opt]
frame #12: 0x00000001154e5fad LLDB`lldb_private::IRExecutionUnit::MemoryManager::GetSymbolAddressAndPresence(...) at IRExecutionUnit.cpp:1063:31 [opt]

[JDevlieghere]

@swift-ci please test