refactor: Remove LLVMRustInsertPrivateGlobal and define_private_global

Since it can easily be implemented using the existing LLVM C API in
terms of `LLVMAddGlobal` and `LLVMSetLinkage` and `define_private_global`
was only used in one place.

compiler/rustc_codegen_llvm/src/consts.rs

@@ -240,11 +240,13 @@ impl<'ll> CodegenCx<'ll, '_> {
                 let gv = self.define_global(&name, self.val_ty(cv)).unwrap_or_else(|| {
                     bug!("symbol `{}` is already defined", name);
                 });
-                llvm::set_linkage(gv, llvm::Linkage::PrivateLinkage);
                 gv
             }
-            _ => self.define_private_global(self.val_ty(cv)),
+            _ => self.define_global("", self.val_ty(cv)).unwrap_or_else(|| {
+                bug!("anonymous global symbol is already defined");
+            }),
         };
+        llvm::set_linkage(gv, llvm::Linkage::PrivateLinkage);
         llvm::set_initializer(gv, cv);
         set_global_alignment(self, gv, align);
         llvm::set_unnamed_address(gv, llvm::UnnamedAddr::Global);

compiler/rustc_codegen_llvm/src/declare.rs

@@ -230,13 +230,6 @@ impl<'ll, CX: Borrow<SCx<'ll>>> GenericCx<'ll, CX> {
         }
     }
 
-    /// Declare a private global
-    ///
-    /// Use this function when you intend to define a global without a name.
-    pub(crate) fn define_private_global(&self, ty: &'ll Type) -> &'ll Value {
-        unsafe { llvm::LLVMRustInsertPrivateGlobal(self.llmod(), ty) }
-    }
-
     /// Gets declared value by name.
     pub(crate) fn get_declared_value(&self, name: &str) -> Option<&'ll Value> {
         debug!("get_declared_value(name={:?})", name);

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1954,7 +1954,6 @@ unsafe extern "C" {
         NameLen: size_t,
         T: &'a Type,
     ) -> &'a Value;
-    pub(crate) fn LLVMRustInsertPrivateGlobal<'a>(M: &'a Module, T: &'a Type) -> &'a Value;
     pub(crate) fn LLVMRustGetNamedValue(
         M: &Module,
         Name: *const c_char,