Improve performance when parsing symbol tables in mach-o files.

Some larger projects were loading quite slowly with the current LLDB on macOS and macOS simulator builds. I did some instrument traces and found 3 main culprits:
- a LLDB timer that was put into a function that was called too often
- a std::set that was keeping track of the address of symbols that were already added
- a unnamed function generator in ObjectFile that was going slow due to allocations

In order to see this in action I ran the latest LLDB on a large application with many frameworks using the following method:

(lldb) script import time; start_time = time.perf_counter()
(lldb) file Large.app
(lldb) script print(time.perf_counter() - start_time)

I first range "sudo purge" to clear the system file caches to simulate a cold startup of the debugger, followed by two iterations with warm file caches.

Prior to this fix I was seeing the following timings:

17.68 (cold)
14.56 (warm 1)
14.52 (warm 2)

After this fix I was seeing:

11.32 (cold)
8.43 (warm 1)
8.49 (warm 2)

Differential Revision: https://reviews.llvm.org/D103504

lldb/source/Core/Module.cpp

@@ -440,8 +440,6 @@ CompUnitSP Module::GetCompileUnitAtIndex(size_t index) {
 
 bool Module::ResolveFileAddress(lldb::addr_t vm_addr, Address &so_addr) {
   std::lock_guard<std::recursive_mutex> guard(m_mutex);
-  LLDB_SCOPED_TIMERF("Module::ResolveFileAddress (vm_addr = 0x%" PRIx64 ")",
-                     vm_addr);
   SectionList *section_list = GetSectionList();
   if (section_list)
     return so_addr.ResolveAddressUsingFileSections(vm_addr, section_list);