Improve natvis for llvm::SmallString so that it correctly displays only the valid portion of the string

[amaiorano]

Summary

The usual method, and the one employed before my change, of displaying strings in natvis is to make use of the "<variable>,s" format specifier; however, this method only works for null-terminated strings. My fix here is to use the "<pointer>,[size]" format specifier to display a bounded array, and then cast it to "const char*", which in the MSVC debugger has the desired effect of rendering the character array as a string.

[amaiorano]

Goes from:

to:

Similarly for the StringView dialog (when you click on the magnifying glass).

[amaiorano]

Added "na" format specifier to not show the pointer address to the string, just like the original visualizer did. Updated screenshot of what it looks like forthcoming...

[amaiorano]

Updated screenshot:

[zturner]

Looks great, thanks

llvm/trunk/utils/LLVMVisualizers/llvm.natvis

<Expand>
       <Item Name="[size]">Length</Item>
       <ArrayItems>
         <Size>Length</Size>
         <ValuePointer>Data</ValuePointer>
       </ArrayItems>
     </Expand>
   </Type>
   <Type Name="llvm::SmallString&lt;*&gt;">
-    <DisplayString>{BeginX,s}</DisplayString>
-    <StringView>BeginX,s</StringView>
+    <DisplayString>{(const char*)BeginX,[(char*)EndX - (char*)BeginX] na}</DisplayString>
+    <StringView>(const char*)BeginX,[(char*)EndX - (char*)BeginX]</StringView>
     <Expand>
       <Item Name="[size]">(char*)EndX - (char*)BeginX</Item>
       <Item Name="[capacity]">(char*)CapacityX - (char*)BeginX</Item>
       <ArrayItems>
         <Size>(char*)EndX - (char*)BeginX</Size>
         <ValuePointer>(char*)BeginX</ValuePointer>
       </ArrayItems>
     </Expand>