Correcting some comments in PdbIndex.cpp [NFC]

ICF can cause multiple symbols to start at the same virtual address.
I plan to handle this shortly, but I wanted to correct the comment for
now.

Deleted an obsolete comment about adjusting the offset for the magic
number at the beginning of the debug info stream.  This adjustment is
handled at a lower level now.

llvm-svn: 355943

lldb/source/Plugins/SymbolFile/NativePDB/PdbIndex.cpp

@@ -132,9 +132,8 @@ void PdbIndex::BuildAddrToSymbolMap(CompilandIndexItem &cci) {
 
     PdbCompilandSymId cu_sym_id(modi, iter.offset());
 
-    // If the debug info is incorrect, we could have multiple symbols with the
-    // same address.  So use try_emplace instead of insert, and the first one
-    // will win.
+    // It's rare, but we could have multiple symbols with the same address
+    // because of identical comdat folding.  Right now, the first one will win.
     cci.m_symbols_by_va.insert(std::make_pair(va, PdbSymUid(cu_sym_id)));
   }
 }
@@ -186,8 +185,6 @@ std::vector<SymbolAndUid> PdbIndex::FindSymbolsByVa(lldb::addr_t va) {
 }
 
 CVSymbol PdbIndex::ReadSymbolRecord(PdbCompilandSymId cu_sym) const {
-  // We need to subtract 4 here to adjust for the codeview debug magic
-  // at the beginning of the debug info stream.
   const CompilandIndexItem *cci = compilands().GetCompiland(cu_sym.modi);
   auto iter = cci->m_debug_stream.getSymbolArray().at(cu_sym.offset);
   lldbassert(iter != cci->m_debug_stream.getSymbolArray().end());