Fix duplicate $td$ typedesc var for const type alias

[senuriwick]

Purpose

Compiling a module containing a type definition that aliases directly to a constant fails with a generic, unhelpful error.

Fixes #44586

Approach

The fix tracks generated typedesc variable names per module in createTypedescVariable and skips creation if a variable with that name already exists, with the tracking reset per module (not per type-definition or per statement list, so as not to disturb the existing, narrower typedescList batching) deduping by generated name rather than by BType object identity, since the two colliding types are structurally related but not the same Java object.

Samples

import ballerina/io;

public const MAP = {
    "foo" : "bar"
};

public type Alias MAP;

public function main() {
    io:println("hello world");
}

Remarks

The current fix dedupes by generated name within a module. This mirrors an existing name-based guard in the same method, but in principle two distinct types could theoretically share a generated name (e.g. via import-shadowing of simple names). This wasn't observed as an issue and is considered out of scope for this fix; flagging for awareness.

Check List

• Read the Contributing Guide
• Added necessary tests
  • Unit Tests

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