[Improvement]: Properly distinguish between type descriptor and type in runtime

[heshanpadmasiri]

Description

In current implementation both type descriptor as well as type is represented by BTypes/Types. However as far the spec is concerned these are separate concepts.

Describe your problem(s)

Combining type descritptor with type introduce several performance problems. For instance consider the fallowing where T is any ballerina type,

function foo() {
       T val = intiValue();
}

For each invocation of foo we must resolve type descriptor of T separately since things like default values could change from invocation to invocation. However type of T does not. But by combining type descritptor and type of T together we anyway initialise "both" type and type descriptor for each invocation.

This limits the potential to reuse type check results from previous invocations and add extra overhead in having to resolve the type for each invocation.

Describe your solution(s)

With 40511 we can treat the semtype part as the "type" and "BType" part as the type descriptor. Each type descriptor will have a semtype (which may be shared between instances, and lazily initialised). Initially we can handle this enitirely at runtime (which will add runtime overhead of canonicalization of types). Later we can do the canonicalization at compile time

Related area

-> Runtime

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[heshanpadmasiri]

Most of the offending new typedesc creations should be fixed with #38844. Will reopen this if issue persists.