[WebAssembly] Fix assert in lookup of section symbols

[aardappel]

Summary

Fixes assert in: https://bugs.llvm.org/show_bug.cgi?id=49036

getWasmSection creates sections if they don't exist, but doesn't add them to the Symbols table. This may cause problems in subsequent calls to getOrCreateSymbol which checks this table, the calls createSymbol assuming it doesn't exist, which then checks UsedNames and finds out it does exist, causing an assert on trying to rename a non-temp symbol.

I tried also fixing the somewhat unintuitive forced suffixing (adding 0), but it turns out that WasmObjectWriter currently assumes these section symbols are unique, so that may have to be a separate fix.

Also worth noting is that getWasmSection calling createSymbol may not be correct to start with, given that createSymbol seems to assume it is creating non-section symbols. But again, for a future fix.

Related: where some of this was introduced: https://github.com/llvm/llvm-project/commit/8d396acac3bc21f688ac707bb42e4698dbdcab7e

[sbc100]

llvm/test/MC/WebAssembly/section-symbol.s

10

This does seem rather odd. It seems like this should be a error... if we are going to add a test for this seemly broken behaviour, perhaps we should add a TODO with a link to a bug to address it?

[aardappel]

llvm/test/MC/WebAssembly/section-symbol.s

10

created bug and TODO: https://bugs.llvm.org/show_bug.cgi?id=49252

[aardappel]

Added bug todo

llvm/lib/MC/MCContext.cpp

auto IterBool = WasmUniquingMap.insert(
       std::make_pair(WasmSectionKey{Section.str(), Group, UniqueID}, nullptr));
   auto &Entry = *IterBool.first;
   if (!IterBool.second)
     return Entry.second;
 
   StringRef CachedName = Entry.first.SectionName;
 
   MCSymbol *Begin = createSymbol(CachedName, true, false);
+  Symbols[Begin->getName()] = Begin;
   cast<MCSymbolWasm>(Begin)->setType(wasm::WASM_SYMBOL_TYPE_SECTION);
 
   MCSectionWasm *Result = new (WasmAllocator.Allocate())
       MCSectionWasm(CachedName, Kind, GroupSym, UniqueID, Begin);
   Entry.second = Result;
 
   auto *F = new MCDataFragment();
   Result->getFragmentList().insert(Result->begin(), F);

llvm/test/MC/WebAssembly/section-symbol.s

+# RUN: llvm-mc -triple=wasm32 < %s | FileCheck %s
+
+# check that we can refer to section symbols of other sections.
+# getWasmSection currently forces the section symbol to have a suffix.
+
+# TODO: fix the 0-suffix: https://bugs.llvm.org/show_bug.cgi?id=49252
+
+    .section    .debug_abbrev,"",@
+    .int8       1
+    .section    .debug_info,"",@
+    .int32      .debug_abbrev0
+
+# CHECK:         .section    .debug_abbrev,"",@
+# CHECK-NEXT:    .int8       1
+# CHECK-NEXT:    .section    .debug_info,"",@
+# CHECK-NEXT:    .int32      .debug_abbrev0