Fix incorrect form test in SymbolFileDWARF

[labath]

Summary

We were checking whether an attribute is in block form by getting the block data pointer, which
was not correct as the pointer be null even if the attribute is in block form. Other places in
the file already use the correct test.

To make this work, I've needed to add DW_FORM_exprlock to the list of "block" forms, which seems
correct as that is how we are parsing it.

[clayborg]

Looks good.

lldb/trunk/source/Plugins/SymbolFile/DWARF/DWARFFormValue.cpp

 }
 
 
 bool
 DWARFFormValue::IsBlockForm(const dw_form_t form)
 {
     switch (form)
     {
+    case DW_FORM_exprloc:
     case DW_FORM_block:
     case DW_FORM_block1:
     case DW_FORM_block2:
     case DW_FORM_block4:
         return true;
     }
     return false;
 }

lldb/trunk/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp

if ((tag == DW_TAG_variable) ||
                                 }
                             }
                         }
                         break;
                     case DW_AT_location:
                         {
                             location_is_const_value_data = false;
                             has_explicit_location = true;
-                            if (form_value.BlockData())
+                            if (DWARFFormValue::IsBlockForm(form_value.Form()))
                             {
                                 const DWARFDataExtractor& debug_info_data = get_debug_info_data();
 
                                 uint32_t block_offset = form_value.BlockData() - debug_info_data.GetDataStart();
                                 uint32_t block_length = form_value.Unsigned();
                                 location.CopyOpcodeData(module, get_debug_info_data(), block_offset, block_length);
                             }
                             else