[clang][NFC] Small mangler cleanups

[urnathan]

Summary

In working on a module mangling problem I noticed a few cleanups to the mangler.

1. Use 'if (auto x = ...' idiom in a couple of places.

2. I noticed both 'isFileContext' and 'isNamespace || isTranslationUnit' synonyms. Let's use the former.

3. The control flow in the seqId mangling was misordered. Let's channel Count von Count. Also fix the inconsistent bracing.

[aaron.ballman]

LGTM aside from a few style nits.

clang/lib/AST/ItaniumMangle.cpp

662

698

clang/lib/AST/ItaniumMangle.cpp

bool ItaniumMangleContextImpl::isUniqueInternalLinkageDecl(
 
   if (isInternalLinkageDecl(ND))
     return true;
 
   return false;
 }
 
 bool ItaniumMangleContextImpl::shouldMangleCXXName(const NamedDecl *D) {
-  const FunctionDecl *FD = dyn_cast<FunctionDecl>(D);
-  if (FD) {
+  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
     LanguageLinkage L = FD->getLanguageLinkage();
     // Overloadable functions need mangling.
     if (FD->hasAttr<OverloadableAttr>())
       return true;
 
     // "main" is not mangled.
     if (FD->isMain())
       return false;
if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
     if (L == CLanguageLinkage)
       return false;
   }
 
   // Otherwise, no mangling is done outside C++ mode.
   if (!getASTContext().getLangOpts().CPlusPlus)
     return false;
 
-  const VarDecl *VD = dyn_cast<VarDecl>(D);
-  if (VD && !isa<DecompositionDecl>(D)) {
+  if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
+    // Decompositions are mangled.
+    if (isa<DecompositionDecl>(VD))
+      return true;
+
     // C variables are not mangled.
     if (VD->isExternC())
       return false;
 
-    // Variables at global scope with non-internal linkage are not mangled
+    // Variables at global scope with non-internal linkage are not mangled.
     const DeclContext *DC = getEffectiveDeclContext(D);
     // Check for extern variable declared locally.
     if (DC->isFunctionOrMethod() && D->hasLinkage())
-      while (!DC->isNamespace() && !DC->isTranslationUnit())
+      while (!DC->isFileContext())
         DC = getEffectiveParentContext(DC);
     if (DC->isTranslationUnit() && D->getFormalLinkage() != InternalLinkage &&
         !CXXNameMangler::shouldHaveAbiTags(*this, VD) &&
-        !isa<VarTemplateSpecializationDecl>(D))
+        !isa<VarTemplateSpecializationDecl>(VD))
       return false;
   }
 
   return true;
 }
 
 void CXXNameMangler::writeAbiTags(const NamedDecl *ND,
                                   const AbiTagList *AdditionalAbiTags) {
void CXXNameMangler::mangleTemplateParameter(unsigned Depth, unsigned Index) {
   if (Depth != 0)
     Out << 'L' << (Depth - 1) << '_';
   if (Index != 0)
     Out << (Index - 1);
   Out << '_';
 }
 
 void CXXNameMangler::mangleSeqID(unsigned SeqID) {
-  if (SeqID == 1)
+  if (SeqID == 0) {
+    // Nothing.
+  } else if (SeqID == 1) {
     Out << '0';
-  else if (SeqID > 1) {
+  } else {
     SeqID--;
 
     // <seq-id> is encoded in base-36, using digits and upper case letters.
     char Buffer[7]; // log(2**32) / log(36) ~= 7
     MutableArrayRef<char> BufferRef(Buffer);
     MutableArrayRef<char>::reverse_iterator I = BufferRef.rbegin();
 
     for (; SeqID != 0; SeqID /= 36) {