Auto merge of #67759 - nikic:llvm-10, r=Mark-Simulacrum

Update to LLVM 10

LLVM 10 is going to be branched soon, so it's a good time to start finding all those tasty new miscompiles and performance regressions ;)

Status:

 * Preparation split off into #67900.
 * Optimization regressions:
   * [x] https://bugs.llvm.org/show_bug.cgi?id=44419 => https://reviews.llvm.org/D72048 has landed.
   * [x] https://bugs.llvm.org/show_bug.cgi?id=44423 => https://reviews.llvm.org/D72060 has landed.
   * [x] https://reviews.llvm.org/D72169 submitted.
   * [ ] https://bugs.llvm.org/show_bug.cgi?id=44461 reported. https://reviews.llvm.org/D72420 submitted, but unlikely eligible for LLVM 10.
 * Compile-time regressions:
   * [x] GlobalOpt regression identified. ~~fhahn proposed https://reviews.llvm.org/D72214.~~ fhahn has [reverted](llvm/llvm-project@192cce1) the patch.
   * [ ] Even with the revert, there are [large regressions](https://perf.rust-lang.org/compare.html?start=760ce94c69ca510d44087291c311296f6d9ccdf5&end=4e84f97d76e694bb9f59039f5bdeb6d8bca46d14).
 * Assertion failures / infinite loops:
   * [x] https://bugs.llvm.org/show_bug.cgi?id=44600 => https://reviews.llvm.org/D73135, https://reviews.llvm.org/D73854 and https://reviews.llvm.org/D73908 have landed and been cherry-picked to the 10.x branch.
   * [x] https://bugs.llvm.org/show_bug.cgi?id=44835 => https://reviews.llvm.org/D74278 has landed and been cherry-picked.

.gitmodules

@@ -37,7 +37,7 @@
 [submodule "src/llvm-project"]
 	path = src/llvm-project
 	url = https://github.com/rust-lang/llvm-project.git
-	branch = rustc/9.0-2019-12-19
+	branch = rustc/10.0-2020-05-05
 [submodule "src/doc/embedded-book"]
 	path = src/doc/embedded-book
 	url = https://github.com/rust-embedded/book.git

src/bootstrap/native.rs

@@ -520,6 +520,10 @@ impl Step for Lld {
                 .define("LLVM_TABLEGEN_EXE", llvm_config.with_file_name("llvm-tblgen"));
         }
 
+        // Explicitly set C++ standard, because upstream doesn't do so
+        // for standalone builds.
+        cfg.define("CMAKE_CXX_STANDARD", "14");
+
         cfg.build();
 
         t!(File::create(&done_stamp));