Rollup merge of rust-lang#52322 - varkor:llvm-7-rebuild-trigger, r=alexcrichton

Update llvm-rebuild-trigger in light of LLVM 7 upgrade

Not triggering a LLVM rebuild [since the LLVM 7 upgrade](rust-lang#51966 (comment)) causes builds of rustc to fail.

r? @alexcrichton

Author: kennytm

src/rustllvm/llvm-rebuild-trigger

@@ -1,4 +1,4 @@
 # If this file is modified, then llvm will be (optionally) cleaned and then rebuilt.
 # The actual contents of this file do not matter, but to trigger a change on the
 # build bots then the contents should be changed so git updates the mtime.
-2018-05-18
+2018-07-12