Run name-anon-globals after LTO passes as well

If we're going to emit bitcode (through ThinLTOBuffer), then we
need to ensure that anon globals are named. This was already done
after optimization passes, but also has to happen after LTO passes,
as we always emit the final result in a ThinLTO-compatible manner.

Fixes #51947.

src/test/ui/issue-51947.rs

@@ -0,0 +1,17 @@
+// compile-pass
+
+#![crate_type = "lib"]
+#![feature(linkage)]
+
+// MergeFunctions will merge these via an anonymous internal
+// backing function, which must be named if ThinLTO buffers are used
+
+#[linkage = "weak"]
+pub fn fn1(a: u32, b: u32, c: u32) -> u32 {
+    a + b + c
+}
+
+#[linkage = "weak"]
+pub fn fn2(a: u32, b: u32, c: u32) -> u32 {
+    a + b + c
+}