Error instead of crash if users write silly things in aliases.

Author: threeifbyair

regression-tests/pure2-bugfix-for-invalid-alias.cpp2

@@ -0,0 +1,3 @@
+outer: type = {
+    x: requires true == 42;
+}

regression-tests/test-results/pure2-bugfix-for-invalid-alias.cpp2.output

@@ -0,0 +1,3 @@
+pure2-bugfix-for-invalid-alias.cpp2...
+pure2-bugfix-for-invalid-alias.cpp2(2,25): error: invalid alias declaration - expected 'type', 'namespace', or a type-id after ':'
+

source/parse.h

@@ -9835,10 +9835,13 @@ class parser
             a->initializer = std::move(e);
         }
 
-        //  Anything else shouldn't be possible
+        //  Anything else is illegal
         else {
-            assert(false && "ICE: should be unreachable - invalid alias declaration");
-            return {};
+            errors.emplace_back(
+                curr().position(),
+                "invalid alias declaration - expected 'type', 'namespace', or a type-id after ':'"
+            );
+        return {};
         }
 
         //  And the final ceremonial semicolon