[WebAssembly] Stabilize custom section order

It currently depends on the StringMap iteration order, which is not
guaranteed to be deterministic. Use MapVector to stabilize the order.

Author: MaskRay

lld/test/wasm/custom-sections.ll

@@ -15,8 +15,8 @@ entry:
 !wasm.custom_sections = !{ !0 }
 
 ; CHECK:        - Type:            CUSTOM
-; CHECK-NEXT:     Name:            green
-; CHECK-NEXT:     Payload:         '626172717578'
-; CHECK-NEXT:   - Type:            CUSTOM
 ; CHECK-NEXT:     Name:            red
 ; CHECK-NEXT:     Payload:         6578747261666F6F
+; CHECK-NEXT:   - Type:            CUSTOM
+; CHECK-NEXT:     Name:            green
+; CHECK-NEXT:     Payload:         '626172717578'

lld/test/wasm/debug-undefined-fs.s

@@ -31,7 +31,7 @@ _start:
     .int32 undef
     .int32 .Ld
 
-# CHECK:          Name:            .debug_info
-# CHECK-NEXT:     Payload:         02000000FFFFFFFF00000000
 # CHECK:          Name:            .debug_int
 # CHECK-NEXT:     Payload:         '01000000'
+# CHECK:          Name:            .debug_info
+# CHECK-NEXT:     Payload:         02000000FFFFFFFF00000000

lld/test/wasm/section-symbol-relocs.yaml

@@ -34,18 +34,18 @@ Sections:
         Flags:           [ BINDING_LOCAL ]
 ...
 
-# CHECK:        Name:            green
-# CHECK-NEXT:   Payload:         626172717578AA0600000003000000
 # CHECK:        Name:            red
 # CHECK-NEXT:   Payload:         666F6FBB0000000000000000
+# CHECK:        Name:            green
+# CHECK-NEXT:   Payload:         626172717578AA0600000003000000
 
 # RELOC:        Relocations:
 # RELOC-NEXT:       - Type:            R_WASM_SECTION_OFFSET_I32
-# RELOC-NEXT:         Index:           0
+# RELOC-NEXT:         Index:           1
 # RELOC-NEXT:         Offset:          0x7
 # RELOC-NEXT:         Addend:          6
 # RELOC-NEXT:       - Type:            R_WASM_SECTION_OFFSET_I32
-# RELOC-NEXT:         Index:           1
+# RELOC-NEXT:         Index:           0
 # RELOC-NEXT:         Offset:          0xB
 # RELOC-NEXT:         Addend:          3
 # RELOC-NEXT:    Name:            green

lld/wasm/Writer.cpp

@@ -22,6 +22,7 @@
 #include "lld/Common/Strings.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
@@ -112,7 +113,7 @@ class Writer {
   uint64_t fileSize = 0;
 
   std::vector<WasmInitEntry> initFunctions;
-  llvm::StringMap<std::vector<InputChunk *>> customSectionMapping;
+  llvm::MapVector<StringRef, std::vector<InputChunk *>> customSectionMapping;
 
   // Stable storage for command export wrapper function name strings.
   std::list<std::string> commandExportWrapperNames;
@@ -162,7 +163,7 @@ void Writer::calculateCustomSections() {
 void Writer::createCustomSections() {
   log("createCustomSections");
   for (auto &pair : customSectionMapping) {
-    StringRef name = pair.first();
+    StringRef name = pair.first;
     LLVM_DEBUG(dbgs() << "createCustomSection: " << name << "\n");
 
     OutputSection *sec = make<CustomSection>(std::string(name), pair.second);