Update source map names field in prolog/epilog

 WebAssembly#8068 failed to update `prologLocation` and `epilogLocation`, which
caused the CI failure: https://app.circleci.com/pipelines/github/emscripten-core/emscripten/47832/workflows/ea0292aa-124d-4a3f-b988-0a96823e9bcd/jobs/1089017/tests

This updates them properly.

Author: aheejin

src/wasm/wasm-binary.cpp

@@ -1245,14 +1245,20 @@ void WasmBinaryWriter::writeSourceMapProlog() {
     std::map<Index, Index> oldToNewIndex;
 
     // Collect all used symbol name indexes.
-    for (auto& func : wasm->functions) {
-      for (auto& [_, location] : func->debugLocations) {
+    auto prepareIndexMap =
+      [&](std::optional<Function::DebugLocation>& location) {
         if (location && location->symbolNameIndex) {
           uint32_t oldIndex = *location->symbolNameIndex;
           assert(oldIndex < wasm->debugInfoSymbolNames.size());
           oldToNewIndex[oldIndex] = 0; // placeholder
         }
+      };
+    for (auto& func : wasm->functions) {
+      for (auto& [_, location] : func->debugLocations) {
+        prepareIndexMap(location);
       }
+      prepareIndexMap(func->prologLocation);
+      prepareIndexMap(func->epilogLocation);
     }
 
     // Create the new list of names and the mapping from old to new indices.
@@ -1263,13 +1269,19 @@ void WasmBinaryWriter::writeSourceMapProlog() {
     }
 
     // Update all debug locations to point to the new indices.
-    for (auto& func : wasm->functions) {
-      for (auto& [_, location] : func->debugLocations) {
+    auto updateIndexMap =
+      [&](std::optional<Function::DebugLocation>& location) {
         if (location && location->symbolNameIndex) {
           uint32_t oldIndex = *location->symbolNameIndex;
           location->symbolNameIndex = oldToNewIndex[oldIndex];
         }
+      };
+    for (auto& func : wasm->functions) {
+      for (auto& [_, location] : func->debugLocations) {
+        updateIndexMap(location);
       }
+      updateIndexMap(func->prologLocation);
+      updateIndexMap(func->epilogLocation);
     }
 
     // Replace the old symbol names with the new, pruned list.

test/lit/source-map-names.wast

@@ -5,11 +5,12 @@
 ;; After --remove-unused-module-elements, the output source map's 'names' field
 ;; should NOT contain 'unused'
 
-;; OUT-MAP: "names":["used","used2"]
+;; OUT-MAP: "names":["used","used2","usedProlog"]
 
 (module
   (export "used" (func $used))
   (export "used2" (func $used2))
+  (export "usedProlog" (func $usedProlog))
 
   (func $unused
     ;;@ src.cpp:1:1:unused
@@ -29,4 +30,11 @@
     ;;@ src.cpp:3:1:used2
     (nop)
   )
+
+  ;; OUT-WAST:      ;;@ src.cpp:4:1:usedProlog
+  ;; OUT-WAST-NEXT: (func
+  ;;@ src.cpp:4:1:usedProlog
+  (func $usedProlog
+    (nop)
+  )
 )