Update the minimum external LLVM to 8

LLVM 8 was released on March 20, 2019, over a year ago.

src/test/debuginfo/generator-objects.rs

@@ -1,7 +1,6 @@
 // ignore-tidy-linelength
 
-// Require LLVM with DW_TAG_variant_part and a gdb that can read it.
-// min-system-llvm-version: 8.0
+// Require a gdb that can read DW_TAG_variant_part.
 // min-gdb-version: 8.2
 
 // compile-flags:-g