CodeGen: handle blocks correctly when inalloca'ed

[compnerd]

Summary

When using blocks with C++ on Windows x86, it is possible to have the
block literal be pushed into the inalloca'ed parameters. Teach IRGen to
handle the case properly by extracting the block literal from the
inalloca parameter. This fixes the use of blocks with C++ on Windows
x86.

[smeenai]

Patch is missing context.

lib/CodeGen/CGDecl.cpp

1851

This comment needs to be updated.

[compnerd]

Update comment

[rjmccall]

I hate inalloca so much. Okay.

[compnerd]

SVN r325724

lib/CodeGen/CGDecl.cpp

   // Use better IR generation for certain implicit parameters.
   if (auto IPD = dyn_cast<ImplicitParamDecl>(&D)) {
     // The only implicit argument a block has is its literal.
-    // We assume this is always passed directly.
+    // This may be passed as an inalloca'ed value on Windows x86.
     if (BlockInfo) {
-      setBlockContextParameter(IPD, ArgNo, Arg.getDirectValue());
+      llvm::Value *V = Arg.isIndirect()
+                           ? Builder.CreateLoad(Arg.getIndirectAddress())
+                           : Arg.getDirectValue();
+      setBlockContextParameter(IPD, ArgNo, V);
       return;
     }
   }

test/CodeGenCXX/block-inalloca.cpp

+// RUN: %clang_cc1 -triple i686-unknown-windows-msvc -fblocks -emit-llvm -o - %s | FileCheck %s
+
+struct S {
+  S(const struct S &) {}
+};
+
+void (^b)(S) = ^(S) {};
+
+// CHECK: [[DESCRIPTOR:%.*]] = getelementptr inbounds <{ i8*, %struct.S, [3 x i8] }>, <{ i8*, %struct.S, [3 x i8] }>* %0, i32 0, i32 0
+// CHECK: load i8*, i8** [[DESCRIPTOR]]
+