[FPGA] [CodeGen][NFC] Enhance Intel FPGA annotation function and optimize code (#14577)

This commit makes several improvements to the
`CodeGenModule::addGlobalIntelFPGAAnnotation` method:

- Updating the comment style to be Doxygen compliant for the
`addGlobalIntelFPGAAnnotation` function.
- Replaced `getAs<RecordType>()` with `castAs<RecordType>()` after
confirming that the variable declaration type is guaranteed to be a
`RecordType`. This change removes the need for a null check and
streamlines the code.
- Modified the base class iteration loop to use a reference (`const auto
&Base`) to avoid unnecessary copies of `CXXBaseSpecifier` objects.

These changes enhance code readability, improve code documentation and
potentially increase performance by avoiding unnecessary dynamic type
checks and copies.

---------

Signed-off-by: Soumi Manna <[email protected]>

Author: smanna12

clang/lib/CodeGen/CodeGenModule.cpp

@@ -5907,22 +5907,19 @@ void CodeGenModule::generateIntelFPGAAnnotation(
   }
 }
 
-/**
- * Adds global Intel FPGA annotations for a given variable declaration.
- * This function handles both simple global variables and fields within
- * structs that are annotated with Intel FPGA attributes. For structs,
- * it recursively visits all fields and base classes to collect annotations.
- *
- * @param VD The variable declaration to annotate.
- * @param GV The LLVM GlobalValue corresponding to the variable declaration.
- */
+/// Adds global Intel FPGA annotations for a given variable declaration.
+/// This function handles both simple global variables and fields within
+/// structs that are annotated with Intel FPGA attributes. For structs,
+/// it recursively visits all fields and base classes to collect annotations.
+/// \param VD The variable declaration to annotate.
+/// \param GV The LLVM GlobalValue corresponding to the variable declaration.
 void CodeGenModule::addGlobalIntelFPGAAnnotation(const VarDecl *VD,
                                                  llvm::GlobalValue *GV) {
   SmallString<256> AnnotStr;
 
   // Handle annotations for fields within a device_global struct.
   if (getLangOpts().IntelFPGA && VD->getType()->isRecordType()) {
-    auto RT = VD->getType()->getAs<RecordType>();
+    auto RT = VD->getType()->castAs<RecordType>();
 
     auto Gen = [&AnnotStr, this](const RecordType *Ty, auto &&Gen) -> void {
       const CXXRecordDecl *RD = cast<CXXRecordDecl>(Ty->getDecl());
@@ -5939,11 +5936,11 @@ void CodeGenModule::addGlobalIntelFPGAAnnotation(const VarDecl *VD,
       }
 
       // Iterate over the base classes of the struct.
-      for (const auto Base : RD->bases()) {
+      for (const auto &Base : RD->bases()) {
         QualType BaseTy = Base.getType();
 
-        if (const auto *BRT = BaseTy->getAs<RecordType>())
-          Gen(BRT, Gen);
+        const auto *BRT = BaseTy->castAs<RecordType>();
+        Gen(BRT, Gen);
       }
     };
     Gen(RT, Gen);