[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#124674)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

This patch migrates uses of PointerUnion::dyn_cast to
dyn_cast_if_present (see the definition of PointerUnion::dyn_cast).
Note that we already have dyn_cast_if_present<T*>(ExplicitInfo)
elsewhere in ClassTemplateSpecializationDecl and
VarTemplateSpecializationDecl, meaning that ExplicitInfo is not
guaranteed to be nonnull in those classes.

Author: kazutakahirata

clang/include/clang/AST/DeclTemplate.h

@@ -2018,7 +2018,8 @@ class ClassTemplateSpecializationDecl : public CXXRecordDecl,
   /// Set the template argument list as written in the sources.
   void
   setTemplateArgsAsWritten(const ASTTemplateArgumentListInfo *ArgsWritten) {
-    if (auto *Info = ExplicitInfo.dyn_cast<ExplicitInstantiationInfo *>())
+    if (auto *Info =
+            dyn_cast_if_present<ExplicitInstantiationInfo *>(ExplicitInfo))
       Info->TemplateArgsAsWritten = ArgsWritten;
     else
       ExplicitInfo = ArgsWritten;
@@ -2032,7 +2033,8 @@ class ClassTemplateSpecializationDecl : public CXXRecordDecl,
 
   /// Gets the location of the extern keyword, if present.
   SourceLocation getExternKeywordLoc() const {
-    if (auto *Info = ExplicitInfo.dyn_cast<ExplicitInstantiationInfo *>())
+    if (auto *Info =
+            dyn_cast_if_present<ExplicitInstantiationInfo *>(ExplicitInfo))
       return Info->ExternKeywordLoc;
     return SourceLocation();
   }
@@ -2780,7 +2782,8 @@ class VarTemplateSpecializationDecl : public VarDecl,
   /// Retrieve the template argument list as written in the sources,
   /// if any.
   const ASTTemplateArgumentListInfo *getTemplateArgsAsWritten() const {
-    if (auto *Info = ExplicitInfo.dyn_cast<ExplicitInstantiationInfo *>())
+    if (auto *Info =
+            dyn_cast_if_present<ExplicitInstantiationInfo *>(ExplicitInfo))
       return Info->TemplateArgsAsWritten;
     return cast<const ASTTemplateArgumentListInfo *>(ExplicitInfo);
   }
@@ -2803,7 +2806,8 @@ class VarTemplateSpecializationDecl : public VarDecl,
 
   /// Gets the location of the extern keyword, if present.
   SourceLocation getExternKeywordLoc() const {
-    if (auto *Info = ExplicitInfo.dyn_cast<ExplicitInstantiationInfo *>())
+    if (auto *Info =
+            dyn_cast_if_present<ExplicitInstantiationInfo *>(ExplicitInfo))
       return Info->ExternKeywordLoc;
     return SourceLocation();
   }
@@ -2813,7 +2817,8 @@ class VarTemplateSpecializationDecl : public VarDecl,
 
   /// Gets the location of the template keyword, if present.
   SourceLocation getTemplateKeywordLoc() const {
-    if (auto *Info = ExplicitInfo.dyn_cast<ExplicitInstantiationInfo *>())
+    if (auto *Info =
+            dyn_cast_if_present<ExplicitInstantiationInfo *>(ExplicitInfo))
       return Info->TemplateKeywordLoc;
     return SourceLocation();
   }

clang/lib/AST/DeclTemplate.cpp

@@ -1496,7 +1496,7 @@ SourceRange VarTemplateSpecializationDecl::getSourceRange() const {
 }
 
 void VarTemplateSpecializationDecl::setExternKeywordLoc(SourceLocation Loc) {
-  auto *Info = ExplicitInfo.dyn_cast<ExplicitInstantiationInfo *>();
+  auto *Info = dyn_cast_if_present<ExplicitInstantiationInfo *>(ExplicitInfo);
   if (!Info) {
     // Don't allocate if the location is invalid.
     if (Loc.isInvalid())
@@ -1509,7 +1509,7 @@ void VarTemplateSpecializationDecl::setExternKeywordLoc(SourceLocation Loc) {
 }
 
 void VarTemplateSpecializationDecl::setTemplateKeywordLoc(SourceLocation Loc) {
-  auto *Info = ExplicitInfo.dyn_cast<ExplicitInstantiationInfo *>();
+  auto *Info = dyn_cast_if_present<ExplicitInstantiationInfo *>(ExplicitInfo);
   if (!Info) {
     // Don't allocate if the location is invalid.
     if (Loc.isInvalid())