mir_transform: implement forced inlining

Adds `#[rustc_force_inline]` which is similar to always inlining but
reports an error if the inlining was not possible, and which always
attempts to inline annotated items, regardless of optimisation levels.
It can only be applied to free functions to guarantee that the MIR
inliner will be able to resolve calls.

compiler/rustc_passes/messages.ftl

@@ -656,6 +656,14 @@ passes_rustc_allow_const_fn_unstable =
 passes_rustc_dirty_clean =
     attribute requires -Z query-dep-graph to be enabled
 
+passes_rustc_force_inline =
+    attribute should be applied to a function
+    .label = not a function definition
+
+passes_rustc_force_inline_coro =
+    attribute cannot be applied to a `async`, `gen` or `async gen` function
+    .label = `async`, `gen` or `async gen` function
+
 passes_rustc_layout_scalar_valid_range_arg =
     expected exactly one integer literal argument