Extract exhaustiveness into its own crate

compiler/rustc_pattern_analysis/src/lib.rs

@@ -0,0 +1,13 @@
+//! Analysis of patterns, notably match exhaustiveness checking.
+
+pub mod constructor;
+pub mod errors;
+pub mod pat;
+pub mod usefulness;
+
+#[macro_use]
+extern crate tracing;
+#[macro_use]
+extern crate rustc_middle;
+
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }