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/Symbol/ObjectFile.cpp

@@ -617,11 +617,13 @@ ObjectFile::GetSymbolTypeFromName(llvm::StringRef name,
 }
 
 ConstString ObjectFile::GetNextSyntheticSymbolName() {
-  StreamString ss;
+  llvm::SmallString<256> name;
+  llvm::raw_svector_ostream os(name);
   ConstString file_name = GetModule()->GetFileSpec().GetFilename();
-  ss.Printf("___lldb_unnamed_symbol%u$$%s", ++m_synthetic_symbol_idx,
-            file_name.GetCString());
-  return ConstString(ss.GetString());
+  ++m_synthetic_symbol_idx;
+  os << "___lldb_unnamed_symbol" << m_synthetic_symbol_idx << "$$"
+     << file_name.GetStringRef();
+  return ConstString(os.str());
 }
 
 std::vector<ObjectFile::LoadableData>