[CIR] Fix assertion failure in CIRGenRecordLayout for Empty Base Optimization

[lanza]

Stack from ghstack (oldest at bottom):

• [CIR] Implement constant expression member pointer casts #2017
• [CIR] Add support for GNU ifunc attribute #2012
• [CIR] Add support for polymorphic typeid with vtable lookup #2007
• [CIR] Add support for ExtVectorBoolType element assignment, comparisons, and logical NOT #2004
• [CIR] Fix RTTI mangling assertion with qualified types #2002
• [CIR] Add support for typeid l-value emission #2001
• -> [CIR] Fix assertion failure in CIRGenRecordLayout for Empty Base Optimization #1999
• [CIR] Add support for ExtVector Bool Type #1998
• [CIR] Implement APValue emission for member function pointers #2016
• [CIR] Implement static variable initialization with guarded init #2015
• [CIR] Implement cast expression classes as l-values #2011
• [CIR] Implement non-virtual thunk generation for multiple inheritance #2006

This patch fixes a critical assertion failure that occurred when accessing
fields in records that use Empty Base Optimization (EBO), particularly
affecting std::unique_ptr, std::shared_ptr, and other standard library types.

Problem:

When processing classes with empty member types combined with inheriting
constructors, CIRGenRecordLayout::getCIRFieldNo() would assert with:
"Assertion `FieldInfo.count(FD) && "Invalid field for record!"' failed"

This occurred because EBO transforms empty-type members into base classes
rather than fields, so they don't exist in the FieldInfo map. The bug was
triggered by the combination of three conditions:

1. A member with an empty struct type (e.g., std::default_delete)
2. Inheriting constructors (using Base::Base)
3. Instantiation of the inherited constructor

Root Cause:

std::unique_ptr internally uses std::tuple<T*, default_delete>, where
default_delete is an empty struct. When std::tuple applies EBO, the empty
deleter is stored as a base class instead of a member field. During code
generation for constructors, CIR tried to initialize this "field" but it
didn't exist in the FieldInfo map, causing an assertion failure.

Solution:

Following the traditional CodeGen pattern from clang/lib/CodeGen, this patch:

1. Adds containsFieldDecl() method to CIRGenRecordLayout (matching
   CGRecordLayout::containsFieldDecl) to check if a field exists in the
   layout before attempting to access it.

2. Adds guard checks before getCIRFieldNo() calls in:

   • CIRGenExpr.cpp::emitLValueForField() - handles field access
   • CIRGenExpr.cpp::emitLValueForFieldInitialization() - handles
     initialization of reference fields
   • CIRGenExprConst.cpp::emitNullConstant() - handles null initialization
   • CIRGenExpr.cpp::emitAddrOfFieldStorage() - removes redundant call

3. When a field doesn't exist in the layout (EBO case), returns appropriate
   fallback values instead of asserting.

Impact:

This fix enables the following previously-broken STL types:

• std::unique_ptr
• std::shared_ptr
• std::function
• std::array<T, N>
• std::any
• std::thread

Testing:

Added clang/test/CIR/CodeGen/ebo-tuple.cpp which tests the specific
scenario that triggered this bug: empty member types with inheriting
constructors.

Verified with:

• Minimal 24-line reproduction case
• std::unique_ptr default construction
• std::shared_ptr default construction

This implementation strictly follows the traditional CodeGen approach from
clang/lib/CodeGen/CGRecordLayout.h and clang/lib/CodeGen/CGExpr.cpp to
maintain consistency with LLVM CodeGen.