Bump llvm to revert accidental unbump when merging the xcode9 branch. (#2828)

In 1690ccbc99 (when merging the Xcode 9 branch
into master), llvm was accidentally debumped from
mono/llvm@6aa74ae572 to
mono/llvm@975e3a6903.

So rebump (and one commit further, to match what we're using in the d15-4
branch).

external/llvm

@@ -1 +1 @@
-Subproject commit 975e3a6903003d7dba1b01f8a3c4a42a55049842
+Subproject commit 21492ec92e255a43bc6b687468f1eb18a635d94e