[Serialization] Make the input/output of a helper lambda clearer

It still has side effects, and it's still a lambda rather than a
helper function because it's not used anywhere else, but `[&]`
captures in helper lambdas make me nervous (as opposed to callback
lambdas).

Author: jrose-apple

lib/Serialization/Deserialization.cpp

@@ -4802,7 +4802,7 @@ class swift::TypeDeserializer {
     decls_block::SILBoxTypeLayout::readRecord(scratch, layoutID, subMapID);
 
     // Get the layout.
-    auto getLayout = [&]() -> SILLayout * {
+    auto getLayout = [this](SILLayoutID layoutID) -> SILLayout * {
       assert(layoutID > 0 && layoutID <= MF.SILLayouts.size()
              && "invalid layout ID");
 
@@ -4822,7 +4822,7 @@ class swift::TypeDeserializer {
       return layout;
     };
 
-    auto layout = getLayout();
+    auto layout = getLayout(layoutID);
     if (!layout)
       return nullptr;