rustbuild: Update LLVM and enable ThinLTO

This commit updates LLVM to fix rust-lang#45511 (https://reviews.llvm.org/D39981) and
also reenables ThinLTO for libtest now that we shouldn't hit rust-lang#45768. This also
opportunistically enables ThinLTO for libstd which was previously blocked
(rust-lang#45661) on test failures related to debuginfo with a presumed cause of rust-lang#45511.

Closes rust-lang#45511

Author: alexcrichton

src/bootstrap/builder.rs

@@ -624,9 +624,7 @@ impl<'a> Builder<'a> {
                 cargo.arg("--release");
             }
 
-            if mode != Mode::Libstd && // FIXME(#45320)
-               mode != Mode::Libtest && // FIXME(#45511)
-               self.config.rust_codegen_units.is_none() &&
+            if self.config.rust_codegen_units.is_none() &&
                self.build.is_rust_llvm(compiler.host)
             {
                 cargo.env("RUSTC_THINLTO", "1");

src/libcompiler_builtins

@@ -1,10 +1,19 @@
 -include ../tools.mk
 
+LOG := $(TMPDIR)/log.txt
+
+# FIXME(#46126) ThinLTO for libstd broke this test
+ifeq (1,0)
 all:
 ifeq ($(TARGET),x86_64-unknown-linux-gnu)
 ifdef SANITIZER_SUPPORT
 	$(RUSTC) -C opt-level=1 -g -Z sanitizer=leak -Z print-link-args leak.rs | grep -q librustc_lsan
-	$(TMPDIR)/leak 2>&1 | grep -q 'detected memory leaks'
+	$(TMPDIR)/leak 2>&1 | tee $(LOG)
+	grep -q 'detected memory leaks' $(LOG)
+endif
 endif
+
+else
+all:
 endif