AST: Add a fast path for IterableDeclContext::getBodyFingerprint()

getParsedMembers() on imported and deserialized contexts has to pull
in all the members. This is wasteful. Imported contexts do not have
fingerprints at all, and fingerprints of deserialized contexts  are
serialized separately.

The fast path handles these cases, only calling down to
getParsedMembers() if the parent FileUnit is a SourceFile.

Author: slavapestov

lib/AST/DeclContext.cpp

@@ -1038,11 +1038,22 @@ IterableDeclContext::castDeclToIterableDeclContext(const Decl *D) {
 }
 
 Optional<Fingerprint> IterableDeclContext::getBodyFingerprint() const {
-  auto mutableThis = const_cast<IterableDeclContext *>(this);
-  return evaluateOrDefault(getASTContext().evaluator,
-                           ParseMembersRequest{mutableThis},
-                           FingerprintAndMembers())
-      .fingerprint;
+  auto fileUnit = dyn_cast<FileUnit>(getAsGenericContext()->getModuleScopeContext());
+  if (!fileUnit)
+    return None;
+
+  if (isa<SourceFile>(fileUnit)) {
+    auto mutableThis = const_cast<IterableDeclContext *>(this);
+    return evaluateOrDefault(getASTContext().evaluator,
+                             ParseMembersRequest{mutableThis},
+                             FingerprintAndMembers())
+        .fingerprint;
+  }
+
+  if (getDecl()->isImplicit())
+    return None;
+
+  return fileUnit->loadFingerprint(this);
 }
 
 /// Return the DeclContext to compare when checking private access in