Move the complex address expression out of DIVariable and into an extra

argument of the llvm.dbg.declare/llvm.dbg.value intrinsics.

Previously, DIVariable was a variable-length field that has an optional
reference to a Metadata array consisting of a variable number of
complex address expressions. In the case of OpPiece expressions this is
wasting a lot of storage in IR, because when an aggregate type is, e.g.,
SROA'd into all of its n individual members, the IR will contain n copies
of the DIVariable, all alike, only differing in the complex address
reference at the end.

By making the complex address into an extra argument of the
dbg.value/dbg.declare intrinsics, all of the pieces can reference the
same variable and the complex address expressions can be uniqued across
the CU, too.
Down the road, this will allow us to move other flags, such as
"indirection" out of the DIVariable, too.

The new intrinsics look like this:
declare void @llvm.dbg.declare(metadata %storage, metadata %var, metadata %expr)
declare void @llvm.dbg.value(metadata %storage, i64 %offset, metadata %var, metadata %expr)

This patch adds a new LLVM-local tag to DIExpressions, so we can detect
and pretty-print DIExpression metadata nodes.

What this patch doesn't do:

This patch does not touch the "Indirect" field in DIVariable; but moving
that into the expression would be a natural next step.

http://reviews.llvm.org/D4919
rdar://problem/17994491

Thanks to dblaikie and dexonsmith for reviewing this patch!

Note: I accidentally committed a bogus older version of this patch previously.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218787 91177308-0d34-0410-b5e6-96231b3b80d8

docs/SourceLevelDebugging.rst

@@ -571,7 +571,7 @@ Local variables
     metadata, ;; Reference to the type descriptor
     i32,      ;; flags
     metadata  ;; (optional) Reference to inline location
-    metadata  ;; (optional) Reference to a complex expression (see below)
+    metadata  ;; (optional) Reference to a complex expression.
   }
 
 These descriptors are used to define variables local to a sub program.  The
@@ -590,7 +590,20 @@ The context is either the subprogram or block where the variable is defined.
 Name the source variable name.  Context and line indicate where the variable
 was defined.  Type descriptor defines the declared type of the variable.
 
-The ``OpPiece`` operator is used for (typically larger aggregate)
+Complex Expressions
+^^^^^^^^^^^^^^^^^^^
+.. code-block:: llvm
+
+  !8 = metadata !{
+    i32,      ;; DW_TAG_expression
+    ...
+  }
+
+Complex expressions describe variable storage locations in terms of
+prefix-notated DWARF expressions. Currently the only supported
+operators are ``DW_OP_plus``, ``DW_OP_deref``, and ``DW_OP_piece``.
+
+The ``DW_OP_piece`` operator is used for (typically larger aggregate)
 variables that are fragmented across several locations. It takes two
 i32 arguments, an offset and a size in bytes to describe which piece
 of the variable is at this location.