Add natvis visualizers for endian packed types

[zturner]

Summary

I only attempt to do unaligned types here, because aligned types start getting pretty ugly.

I don't know if there's a better way to do this. Let me know if you can think of something cleaner.

[zturner]

Apparently you can line break inside of the <DisplayString> tag without messing anything up, so this should be easier to read.

[aaron.ballman]

LGTM, thank you!

[Eugene.Zelenko]

Committed in r267050.

utils/LLVMVisualizers/llvm.natvis

       <Item Name="[underlying]" Condition="hasVal">*(($T1 *)(unsigned char *)storage.buffer)</Item>
     </Expand>
   </Type>
+
+  <!-- Since we're in MSVC, we can assume that the system is little endian.  Therefore
+       the little and native cases just require a cast.  Handle this easy case first. Use
+       a wildcard for the second template argument (the endianness), but we will use a
+       specific value of 0 later on for the big endian to give it priority for being a
+       better match.  -->
+  <Type Name="llvm::support::detail::packed_endian_specific_integral&lt;*,*,1&gt;">
+    <DisplayString>little endian {*(($T1*)(unsigned char *)Value.buffer)}</DisplayString>
+    <Expand>
+      <Item Name="[value]">*(($T1*)(unsigned char *)Value.buffer)</Item>
+      <Item Name="[Raw Bytes]" Condition="sizeof($T1)==1">(unsigned char *)Value.buffer,1</Item>
+      <Item Name="[Raw Bytes]" Condition="sizeof($T1)==2">(unsigned char *)Value.buffer,2</Item>
+      <Item Name="[Raw Bytes]" Condition="sizeof($T1)==4">(unsigned char *)Value.buffer,4</Item>
+      <Item Name="[Raw Bytes]" Condition="sizeof($T1)==8">(unsigned char *)Value.buffer,8</Item>
+    </Expand>
+  </Type>
+  
+  <!-- Now handle the hard case of big endian.  We need to do the swizzling here, but
+       we need to specialize it based on the size of the value type. -->
+  <Type Name="llvm::support::detail::packed_endian_specific_integral&lt;*,0,1&gt;">
+    <DisplayString Condition="sizeof($T1)==1">big endian {*(unsigned char *)Value.buffer}</DisplayString>
+    <DisplayString Condition="sizeof($T1)==2">big endian {(($T1)(*(unsigned char *)Value.buffer) &lt;&lt; 8) | ($T1)(*((unsigned char *)Value.buffer+1))}</DisplayString>
+    <DisplayString Condition="sizeof($T1)==4">big endian {(($T1)(*(unsigned char *)Value.buffer) &lt;&lt; 24) | (($T1)(*((unsigned char *)Value.buffer+1)) &lt;&lt; 16) | (($T1)(*((unsigned char *)Value.buffer+2)) &lt;&lt; 8) | ($T1)(*((unsigned char *)Value.buffer+3))}</DisplayString>
+    <DisplayString Condition="sizeof($T1)==8">big endian {(($T1)(*(unsigned char *)Value.buffer) &lt;&lt; 56) | (($T1)(*((unsigned char *)Value.buffer+1)) &lt;&lt; 48) | (($T1)(*((unsigned char *)Value.buffer+2)) &lt;&lt; 40) | (($T1)(*((unsigned char *)Value.buffer+3)) &lt;&lt; 32) | (($T1)(*((unsigned char *)Value.buffer+4)) &lt;&lt; 24) | (($T1)(*((unsigned char *)Value.buffer+5)) &lt;&lt; 16) | (($T1)(*((unsigned char *)Value.buffer+6)) &lt;&lt; 8) | ($T1)(*((unsigned char *)Value.buffer+7))}</DisplayString>
+    <Expand>
+      <Item Name="[Raw Bytes]" Condition="sizeof($T1)==1">(unsigned char *)Value.buffer,1</Item>
+      <Item Name="[Raw Bytes]" Condition="sizeof($T1)==2">(unsigned char *)Value.buffer,2</Item>
+      <Item Name="[Raw Bytes]" Condition="sizeof($T1)==4">(unsigned char *)Value.buffer,4</Item>
+      <Item Name="[Raw Bytes]" Condition="sizeof($T1)==8">(unsigned char *)Value.buffer,8</Item>
+    </Expand>
+  </Type>
 </AutoVisualizer>