Add methods to create constants

I've been experimenting with transforming the StableMIR to instrument
the code with potential UB checks. The modified body will only
be used by our analysis tool, however, constants in StableMIR must be
backed by rustc constants. Thus, I'm adding a few functions to build
constants, such as building string and other primitives.

compiler/stable_mir/src/ty.rs

@@ -128,13 +128,38 @@ impl Const {
 
     /// Creates an interned usize constant.
     fn try_from_target_usize(val: u64) -> Result<Self, Error> {
-        with(|cx| cx.usize_to_const(val))
+        with(|cx| cx.try_new_const_uint(val.into(), UintTy::Usize))
     }
 
     /// Try to evaluate to a target `usize`.
     pub fn eval_target_usize(&self) -> Result<u64, Error> {
         with(|cx| cx.eval_target_usize(self))
     }
+
+    /// Create a constant that represents a new zero-sized constant of type T.
+    /// Fails if the type is not a ZST or if it doesn't have a known size.
+    pub fn try_new_zero_sized(ty: Ty) -> Result<Const, Error> {
+        with(|cx| cx.try_new_const_zst(ty))
+    }
+
+    /// Build a new constant that represents the given string.
+    ///
+    /// Note that there is no guarantee today about duplication of the same constant.
+    /// I.e.: Calling this function multiple times with the same argument may or may not return
+    /// the same allocation.
+    pub fn from_str(value: &str) -> Const {
+        with(|cx| cx.new_const_str(value))
+    }
+
+    /// Build a new constant that represents the given boolean value.
+    pub fn from_bool(value: bool) -> Const {
+        with(|cx| cx.new_const_bool(value))
+    }
+
+    /// Build a new constant that represents the given unsigned integer.
+    pub fn try_from_uint(value: u128, uint_ty: UintTy) -> Result<Const, Error> {
+        with(|cx| cx.try_new_const_uint(value, uint_ty))
+    }
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]