[Dependency Scanning] Keep track of each imported module's access control

Adds an access control field for each imported module identified. When multiple imports of the same module are found, this keeps track of the most "open" access specifier.

include/swift/DependencyScan/SerializedModuleDependencyCacheFormat.h

@@ -67,6 +67,9 @@ using IsExportedImport = BCFixed<1>;
 using LineNumberField = BCFixed<32>;
 using ColumnNumberField = BCFixed<32>;
 
+/// Access level of an import
+using AccessLevelField = BCFixed<8>;
+
 /// Arrays of various identifiers, distinguished for readability
 using IdentifierIDArryField = llvm::BCArray<IdentifierIDField>;
 using ModuleIDArryField = llvm::BCArray<IdentifierIDField>;
@@ -192,7 +195,8 @@ using ImportStatementLayout =
                    LineNumberField,              // lineNumber
                    ColumnNumberField,            // columnNumber
                    IsOptionalImport,             // isOptional
-                   IsExportedImport              // isExported
+                   IsExportedImport,             // isExported
+                   AccessLevelField              // accessLevel
                    >;
 using ImportStatementArrayLayout =
     BCRecordLayout<IMPORT_STATEMENT_ARRAY_NODE, IdentifierIDArryField>;