fix #61531 swift-symbolgraph-extract crashes when trying to emit the 'Swift' module (#62825)

fix a crash in symbol graph generation caused by an empty SmallVector access while expanding protocol compositions during conformance expansion for extension block symbols

rdar://103322385

Author: theMomax

lib/SymbolGraphGen/SymbolGraphASTWalker.cpp

@@ -214,7 +214,8 @@ bool SymbolGraphASTWalker::walkToDeclPre(Decl *D, CharSourceRange Range) {
           llvm_unreachable("Expected ProtocolDecl or ProtocolCompositionType");
         }
 
-        while (const auto *Comp = UnexpandedCompositions.pop_back_val()) {
+        while (!UnexpandedCompositions.empty()) {
+          const auto *Comp = UnexpandedCompositions.pop_back_val();
           for (const auto &Member : Comp->getMembers()) {
             if (const auto *Proto =
                     dyn_cast_or_null<ProtocolDecl>(Member->getAnyNominal())) {

test/SymbolGraph/Relationships/ConformsTo/Indirect.swift

@@ -35,6 +35,12 @@ extension ES: EQ {
 // EQ : EP
 // EXTERNAL-DAG: "kind": "conformsTo",{{[[:space:]]*}}"source": "s:16ExternalIndirect2EQP",{{[[:space:]]*}}"target": "s:16ExternalIndirect2EPP"
 
+// extension ES : EA (originating from EP : EAB and typealias EAB = EA & EB)
+// EXTENSION-DAG: "kind": "conformsTo",{{[[:space:]]*}}"source": "s:e:s:16ExternalIndirect2ESV0B0E3fooyyF",{{[[:space:]]*}}"target": "s:16ExternalIndirect2EAP"
+
+// extension ES : EB (originating from EP : EAB and typealias EAB = EA & EB)
+// EXTENSION-DAG: "kind": "conformsTo",{{[[:space:]]*}}"source": "s:e:s:16ExternalIndirect2ESV0B0E3fooyyF",{{[[:space:]]*}}"target": "s:16ExternalIndirect2EBP"
+
 // extension ES : EP
 // EXTENSION-DAG: "kind": "conformsTo",{{[[:space:]]*}}"source": "s:e:s:16ExternalIndirect2ESV0B0E3fooyyF",{{[[:space:]]*}}"target": "s:16ExternalIndirect2EPP"
 

test/SymbolGraph/Relationships/ConformsTo/Inputs/ExternalIndirect.swift

@@ -1,4 +1,9 @@
-public protocol EP {
+public protocol EA {}
+public protocol EB {}
+
+public typealias EAB = EA & EB
+
+public protocol EP : EAB {
   func foo()
 }