lto: load bitcode sections by name

Upstream change
llvm/llvm-project@6b539f5eb8 changed
`isSectionBitcode` works and it now only respects `.llvm.lto` sections
instead of also `.llvmbc`, which it says was never intended to be used
for LTO. We instead load sections by name, and sniff for raw bitcode by
hand.

r? @nikic
@rustbot label: +llvm-main

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2322,6 +2322,12 @@ extern "C" {
         len: usize,
         out_len: &mut usize,
     ) -> *const u8;
+    pub fn LLVMRustGetSliceFromObjectDataByName(
+        data: *const u8,
+        len: usize,
+        name: *const u8,
+        out_len: &mut usize,
+    ) -> *const u8;
 
     pub fn LLVMRustLinkerNew(M: &Module) -> &mut Linker<'_>;
     pub fn LLVMRustLinkerAdd(