[clang] Use internal linkage for c23 constexpr vars.

Set `static` storage class specifier for such decls
to have `internal` linkage in produced IR and then
in the object file.
Fix #97830

Author: chestnykh

clang/lib/Parse/ParseDecl.cpp

@@ -4428,10 +4428,15 @@ void Parser::ParseDeclarationSpecifiers(
 
     // constexpr, consteval, constinit specifiers
     case tok::kw_constexpr:
-      if (getLangOpts().C23)
+      if (getLangOpts().C23) {
         Diag(Tok, diag::warn_c23_compat_keyword) << Tok.getName();
-      isInvalid = DS.SetConstexprSpec(ConstexprSpecKind::Constexpr, Loc,
-                                      PrevSpec, DiagID);
+        isInvalid = DS.SetConstexprSpec(ConstexprSpecKind::Constexpr, Loc,
+                                        PrevSpec, DiagID);
+
+        isInvalid = DS.SetStorageClassSpec(Actions, DeclSpec::SCS_static, Loc,
+                                           PrevSpec, DiagID, Policy);
+        isStorageClass = true;
+      }
       break;
     case tok::kw_consteval:
       isInvalid = DS.SetConstexprSpec(ConstexprSpecKind::Consteval, Loc,

clang/test/CodeGen/constexpr-c23-internal-linkage.c

@@ -0,0 +1,18 @@
+/*
+ * RUN: %clang_cc1 -std=c23 -emit-llvm -o - %s | FileCheck %s
+ */
+
+constexpr int var_int = 1;
+constexpr char var_char = 'a';
+constexpr float var_float = 2.5;
+
+const int *p_i = &var_int;
+const char *p_c = &var_char;
+const float *p_f = &var_float;
+
+/*
+CHECK: @var_int = internal constant i32 1{{.*}}
+CHECK: @var_char = internal constant i8 97{{.*}}
+CHECK: @var_float = internal constant float 2.5{{.*}}
+*/
+