Auto merge of #87570 - nikic:llvm-13, r=nagisa

Upgrade to LLVM 13

Work in progress update to LLVM 13. Main changes:

 * InlineAsm diagnostics reported using SrcMgr diagnostic kind are now handled. Previously these used a separate diag handler.
 * Codegen tests are updated for additional attributes.
 * Some data layouts have changed.
 * Switch `#[used]` attribute from `llvm.used` to `llvm.compiler.used` to avoid SHF_GNU_RETAIN flag introduced in https://reviews.llvm.org/D97448, which appears to trigger a bug in older versions of gold.
 * Set `LLVM_INCLUDE_TESTS=OFF` to avoid Python 3.6 requirement.

Upstream issues:

 * ~~https://bugs.llvm.org/show_bug.cgi?id=51210 (InlineAsm diagnostic reporting for module asm)~~ Fixed by 1558bb80c0.
 * ~~https://bugs.llvm.org/show_bug.cgi?id=51476 (Miscompile on AArch64 due to incorrect comparison elimination)~~ Fixed by 81b106584f.
 * https://bugs.llvm.org/show_bug.cgi?id=51207 (Can't set custom section flags anymore). Problematic change reverted in our fork, https://reviews.llvm.org/D107216 posted for upstream revert.
 * https://bugs.llvm.org/show_bug.cgi?id=51211 (Regression in codegen for #83623). This is an optimization regression that we may likely have to eat for this release. The fix for #83623 was based on an incorrect premise, and this needs to be properly addressed in the MergeICmps pass.

The [compile-time impact](https://perf.rust-lang.org/compare.html?start=ef9549b6c0efb7525c9b012148689c8d070f9bc0&end=0983094463497eec22d550dad25576a894687002) is mixed, but quite positive as LLVM upgrades go.

The LLVM 13 final release is scheduled for Sep 21st. The current nightly is scheduled for stable release on Oct 21st.

r? `@ghost`

src/test/ui/llvm-asm/issue-69092.rs

@@ -1,5 +1,7 @@
 // build-fail
 // ignore-emscripten no asm! support
+// The error message differs slightly between LLVM versions
+// min-llvm-version: 13.0
 // Regression test for #69092
 
 #![feature(llvm_asm)]
@@ -7,5 +9,5 @@
 
 fn main() {
     unsafe { llvm_asm!(".ascii \"Xen\0\""); }
-    //~^ ERROR: expected string in '.ascii' directive
+    //~^ ERROR: expected string
 }

src/test/ui/llvm-asm/issue-69092.stderr

@@ -1,5 +1,5 @@
-error: expected string in '.ascii' directive
-  --> $DIR/issue-69092.rs:9:14
+error: expected string
+  --> $DIR/issue-69092.rs:11:14
    |
 LL |     unsafe { llvm_asm!(".ascii \"Xen\0\""); }
    |              ^