[HLSL] Allow EmptyDecl in cbuffer/tbuffer

We do handle EmptyDecls in codegen already as of #124886, but we were
blocking them in Sema. EmptyDecls tend to be caused by extra semicolons
which are not illegal.

Fixes #128238

Author: llvm-beanz

clang/lib/Parse/ParseHLSL.cpp

@@ -30,7 +30,7 @@ static bool validateDeclsInsideHLSLBuffer(Parser::DeclGroupPtrTy DG,
   // Only allow function, variable, record decls inside HLSLBuffer.
   for (DeclGroupRef::iterator I = Decls.begin(), E = Decls.end(); I != E; ++I) {
     Decl *D = *I;
-    if (isa<CXXRecordDecl, RecordDecl, FunctionDecl, VarDecl>(D))
+    if (isa<CXXRecordDecl, RecordDecl, FunctionDecl, VarDecl, EmptyDecl>(D))
       continue;
 
     // FIXME: support nested HLSLBuffer and namespace inside HLSLBuffer.

clang/test/SemaHLSL/cb_error.hlsl

@@ -47,3 +47,15 @@ tbuffer B {
   // expected-error@+1 {{unknown type name 'flaot'}}
   flaot f;
 }
+
+// None of these should produce an error!
+cbuffer EmptyCBuffer {}
+
+cbuffer EmptyDeclCBuffer {
+  ;
+}
+
+cbuffer EmptyDecl2CBuffer {
+  ;
+  int X;
+}