Refactor AddInst-family functions to their own file (#4941)

This in particular uses free functions because it's likely to end up
more consistent with types (versus a wrapper object for InstStore).
Note, this is unlikely to have a performance impact, but if it does, we
can look into related approaches (and we've already discussed using
LTO).

Renames `PendingBlock::AddInst` to `PendingBlock::Add` because
`MakeElementAccessInst` expects the matching name to exist.

toolchain/check/handle_array.cpp

@@ -5,6 +5,7 @@
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
 #include "toolchain/check/handle.h"
+#include "toolchain/check/inst.h"
 #include "toolchain/check/type.h"
 #include "toolchain/parse/node_kind.h"
 
@@ -53,10 +54,10 @@ auto HandleParseNode(Context& context, Parse::ArrayExprId node_id) -> bool {
   bound_inst_id = ConvertToValueOfType(
       context, context.insts().GetLocId(bound_inst_id), bound_inst_id,
       GetSingletonType(context, SemIR::IntLiteralType::SingletonInstId));
-  context.AddInstAndPush<SemIR::ArrayType>(
-      node_id, {.type_id = SemIR::TypeType::SingletonTypeId,
-                .bound_id = bound_inst_id,
-                .element_type_id = element_type_id});
+  AddInstAndPush<SemIR::ArrayType>(context, node_id,
+                                   {.type_id = SemIR::TypeType::SingletonTypeId,
+                                    .bound_id = bound_inst_id,
+                                    .element_type_id = element_type_id});
   return true;
 }