[lldb] Fix mangled name being overwritten on type with DW_AT_specification

The order of the attributes wasn't deterministic, and it just happened
that the DW_AT_linkage_name of the specifier type used to always come
last. Now the algorithm for parsing attributes has changed and made
deterministic, and the current algorithm will parse the attributes of
the specifier TAG first, and specified last.

Make sure to not overwrite the mangled name of a specified type.

rdar://158634690

Author: augusto2112

lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserSwift.cpp

@@ -103,7 +103,12 @@ lldb::TypeSP DWARFASTParserSwift::ParseTypeFromDWARF(const SymbolContext &sc,
           break;
         case llvm::dwarf::DW_AT_linkage_name:
         case llvm::dwarf::DW_AT_MIPS_linkage_name: {
-          mangled_name.SetCString(form_value.AsCString());
+          // This could be a type with DW_AT_specification, in which case,
+          // don't overwrite the mangled name if it's already set (the 
+          // order of the attributes is deterministic, and the "specifier" 
+          // attributes will always come before the specified).
+          if (mangled_name.IsEmpty())
+            mangled_name.SetCString(form_value.AsCString());
           auto HasSpecificationOf = [&](){
             if (has_specification_of)
               return true;