[REG2.067a] Issue 14039 - function is incorrectly inferred as 'pure'

[9rnsr]

https://issues.dlang.org/show_bug.cgi?id=14039

Fixed behavior:
If a nested function is declared inside pure function, it's implicitly marked as pure. It's introduced behavior to fix issue 9148 in #3626.

auto foo() pure {
    int x;
    auto bar() {
        // bar is also marked as pure automatically,
        // because bar can access enclosing pure funciont context.
        x = 1;
    }
}

However, if a nested function is template, pure inference should be preferred than the implicit "pure inheritance".

auto foo() pure {
    int x;
    auto bar()() {   // template function bar is impure by default
        // bar will be forced to get weak purity, only when it's trying
        // to access enclosing pure function context.
        //x = 1;

        // otherwise, bar can call impure functions and then
        // it will be inferred to impure.
        impureFuncCall().
    }
}

While debugging, I found an another purity inference regression since 2.064. This PR will also fix it.
Issue 14049 - [REG2.064] Wrong purity inference for nested lambda

[9rnsr]

@tgehr Could you review the adjusted compiler behavior?

[WalterBright]

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