[HLSL][DirectX] Use a padding types for HLSL buffers.

This change drops the use of the "Layout" type and instead uses explicit
padding throughout the compiler to represent types in HLSL buffers.

There are a few parts to this, though it's difficult to split them up as
they're very interdependent:

1. Refactor HLSLBufferLayoutBuilder to allow us to calculate the padding
   of arbitrary types.
2. Teach Clang CodeGen to use HLSL specific paths for cbuffers when
   generating aggregate copies, array accesses, and structure accesses.
3. Simplify DXILCBufferAccesses such that it directly replaces accesses
   with dx.resource.getpointer rather than recalculating the layout.
4. Basic infrastructure for SPIR-V handling, but the implementation
   itself will need work in follow ups.

Fixes several issues, including #138996, #144573, and #156084.
Resolves #147352.

Author: bogner

clang/lib/CodeGen/CGExpr.cpp

@@ -4641,11 +4641,17 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
                                  LHS.getBaseInfo(), TBAAAccessInfo());
   }
 
-  // The HLSL runtime handle the subscript expression on global resource arrays.
-  if (getLangOpts().HLSL && (E->getType()->isHLSLResourceRecord() ||
-                             E->getType()->isHLSLResourceRecordArray())) {
-    std::optional<LValue> LV =
-        CGM.getHLSLRuntime().emitResourceArraySubscriptExpr(E, *this);
+  // The HLSL runtime handles subscript expressions on global resource arrays
+  // and objects with HLSL buffer layouts.
+  if (getLangOpts().HLSL) {
+    std::optional<LValue> LV;
+    if (E->getType()->isHLSLResourceRecord() ||
+        E->getType()->isHLSLResourceRecordArray()) {
+      LV = CGM.getHLSLRuntime().emitResourceArraySubscriptExpr(E, *this);
+    } else if (E->getType().getAddressSpace() == LangAS::hlsl_constant) {
+      LV = CGM.getHLSLRuntime().emitBufferArraySubscriptExpr(E, *this,
+                                                             EmitIdxAfterBase);
+    }
     if (LV.has_value())
       return *LV;
   }
@@ -5110,6 +5116,11 @@ LValue CodeGenFunction::EmitMemberExpr(const MemberExpr *E) {
     EmitIgnoredExpr(E->getBase());
     return EmitDeclRefLValue(DRE);
   }
+  if (getLangOpts().HLSL &&
+      E->getType().getAddressSpace() == LangAS::hlsl_constant) {
+    // We have an HLSL buffer - emit using HLSL's layout rules.
+    return CGM.getHLSLRuntime().emitBufferMemberExpr(*this, E);
+  }
 
   Expr *BaseExpr = E->getBase();
   // Check whether the underlying base pointer is a constant null.

clang/lib/CodeGen/CGExprAgg.cpp

@@ -2279,6 +2279,10 @@ void CodeGenFunction::EmitAggregateCopy(LValue Dest, LValue Src, QualType Ty,
     }
   }
 
+  if (getLangOpts().HLSL && Ty.getAddressSpace() == LangAS::hlsl_constant)
+    if (CGM.getHLSLRuntime().emitBufferCopy(*this, DestPtr, SrcPtr, Ty))
+      return;
+
   // Aggregate assignment turns into llvm.memcpy.  This is almost valid per
   // C99 6.5.16.1p3, which states "If the value being stored in an object is
   // read from another object that overlaps in anyway the storage of the first