[llvm] [WIP][docs] Strongly typed DIExpression design doc (PR #208541)

[Abid Qadeer via llvm-commits]

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@@ -0,0 +1,239 @@
+# Strongly typed `DIExpression` API
+
+```{contents}
+:local:
+```
+
+## Introduction
+
+The current [`DIExpression`](#diexpression) representation has a number of nice
+properties:
+
+* The core C++ API is just `SmallVector<uint64_t>` and a `get` function.
+  * Code can inspect and edit the representation with no hard dependencies.
+  * The cost to `#include` the support code for this is light on templates and
+    other things which make compilation slow.
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abidh wrote:

I think this could be worded a bit better. In its current form, it could be interpreted that current approach make the compilation slow while it means the opposite.

https://github.com/llvm/llvm-project/pull/208541