[AVR][No Upstream] Use forked LLVM with non-upstreamed fixes

Author: shepmaster

.gitmodules

@@ -42,8 +42,8 @@
 	url = https://github.com/rust-lang-nursery/edition-guide.git
 [submodule "src/llvm-project"]
 	path = src/llvm-project
-	url = https://github.com/rust-lang/llvm-project.git
-	branch = rustc/8.0-2019-03-18
+	url = https://github.com/avr-rust/llvm-project.git
+	branch = avr-rustc/8.0-2019-03-18
 [submodule "src/doc/embedded-book"]
 	path = src/doc/embedded-book
 	url = https://github.com/rust-embedded/book.git