MS Compat: mark globals emitted in read-only sections const

They cannot be written to, so marking them const makes sense and may improve
optimisation.

As a side-effect, SectionInfos has to be moved from Sema to ASTContext.

It also fixes this problem, that occurs when compiling ATL:

  warning LNK4254: section 'ATL' (C0000040) merged into '.rdata' (40000040) with different attributes

The ATL headers are putting variables in a special section that's marked
read-only. However, Clang currently can't model that read-onlyness in the IR.
But, by making the variables const, the section does become read-only, and
the linker warning is avoided.

Differential Revision: http://reviews.llvm.org/D5812

llvm-svn: 219960

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1931,6 +1931,13 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D) {
   GV->setConstant(!NeedsGlobalCtor && !NeedsGlobalDtor &&
                   isTypeConstant(D->getType(), true));
 
+  // If it is in a read-only section, mark it 'constant'.
+  if (const SectionAttr *SA = D->getAttr<SectionAttr>()) {
+    const ASTContext::SectionInfo &SI = Context.SectionInfos[SA->getName()];
+    if ((SI.SectionFlags & ASTContext::PSF_Write) == 0)
+      GV->setConstant(true);
+  }
+
   GV->setAlignment(getContext().getDeclAlign(D).getQuantity());
 
   // Set the llvm linkage type as appropriate.