[mojo-lang] Teach comptime interpreter about LLVM intrinsics.

This teaches the mojo comptime interpreter about LLVM intrinsics,
at least for simple cases involving integer operands and results
(it would be straightforward to extend this to other types now).

This fixes #933 and
Fixes MOCO-138 and Fixes MOCO-504

MODULAR_ORIG_COMMIT_REV_ID: 900128df66da907a0bf61a3b2affb730ef253e17

docs/changelog.md

@@ -16,6 +16,10 @@ what we publish.
 
 ### ⭐️ New
 
+- Mojo can now interpret simple LLVM intrinsics in parameter expressions,
+  enabling things like `count_leading_zeros` to work at compile time:
+  [Issue #933](https://github.com/modularml/mojo/issues/933).
+
 - The VS Code Mojo Debugger now has a `buildArgs` JSON debug configuration
   setting that can be used in conjunction with `mojoFile` to define the build
   arguments when compiling the Mojo file.

stdlib/test/builtin/test_uint.mojo

@@ -14,6 +14,7 @@
 
 from testing import assert_equal, assert_false, assert_not_equal, assert_true
 from sys import bitwidthof
+from bit import count_trailing_zeros
 
 
 def test_simple_uint():
@@ -237,6 +238,13 @@ def test_pos():
     assert_equal(UInt(0).__pos__(), UInt(0))
 
 
+def test_comptime():
+    alias a: UInt = 32
+    # Verify that count_trailing_zeros works at comptime.
+    alias n = count_trailing_zeros(a)
+    assert_equal(n, 5)
+
+
 def main():
     test_simple_uint()
     test_uint_representation()
@@ -260,3 +268,4 @@ def main():
     test_indexer()
     test_comparison()
     test_pos()
+    test_comptime()