Remove rustc_middle::ich::hcx module.

`rustc_middle::ich` is now just a very thin wrapper around
`rustc_middle::ich::hcx`, so the contents of the latter can be moved
into the former.

Author: nnethercote

compiler/rustc_middle/src/ich/hcx.rs compiler/rustc_middle/src/ich.rscompiler/rustc_middle/src/ich/hcx.rs renamed to compiler/rustc_middle/src/ich.rs

@@ -1586,7 +1586,7 @@ dep-bumps = [
 "/compiler/rustc_codegen_llvm/src/debuginfo" =           ["compiler", "debuginfo"]
 "/compiler/rustc_codegen_ssa" =                          ["compiler", "codegen"]
 "/compiler/rustc_middle/src/dep_graph" =                 ["compiler", "incremental", "query-system"]
-"/compiler/rustc_middle/src/ich" =                       ["compiler", "incremental", "query-system"]
+"/compiler/rustc_middle/src/ich.rs" =                    ["compiler", "incremental", "query-system"]
 "/compiler/rustc_middle/src/mir" =                       ["compiler", "mir"]
 "/compiler/rustc_middle/src/traits" =                    ["compiler", "types"]
 "/compiler/rustc_middle/src/query" =                     ["compiler", "query-system"]