FunctionMetadata: introduce cache

FunctionMetadata was being deserialized every time they were
inspected. This commit introduces a cache structure to prevent this
excessive deserializations.

lib/EarlyFunctionAnalysis/FunctionMetadataCache.cpp

@@ -0,0 +1,16 @@
+//
+// This file is distributed under the MIT License. See LICENSE.md for details.
+//
+
+#include "revng/EarlyFunctionAnalysis/FunctionMetadataCache.h"
+
+char FunctionMetadataCachePass::ID = '_';
+
+llvm::AnalysisKey FunctionMetadataCacheAnalysis::Key;
+
+static llvm::RegisterPass<FunctionMetadataCachePass> _("metadata-cache",
+                                                       "Create metadata cache "
+                                                       "to be "
+                                                       "used by later passes",
+                                                       true,
+                                                       true);