fix(libutil/posix-source-accessor.cc): get rid of use-after-move bug (backport #11837)

[mergify]

Naming class member variables the same as constructor arguments is a very slippery slope because of how member variable names get resolved. Compiler is not very helpful here and we need static analysis to forbid this kind of stuff.

The following example illustrates the cause quite well:

struct B {
    B(int) {}
};

struct A {
    A(int b): b([&](){
        return b;
        static_assert(std::is_same_v<decltype(b), int>);
    }()) {
       static_assert(std::is_same_v<decltype(b), int>);
    }
    void member() {
        static_assert(std::is_same_v<decltype(b), B>);
    }
    B b;
};

int main() {
    A(1).member();
}

From N4861 6.5.1 Unqualified name lookup:

In all the cases listed in [basic.lookup.unqual], the scopes are searched
for a declaration in the order listed in each of the respective categories;
name lookup ends as soon as a declaration is found for the name.
If no declaration is found, the program is ill-formed.

In the affected code there was a use-after-move for all accessses in the constructor body, but this UB wasn't triggered.

These types of errors are trivial to catch via clang-tidy's [clang-analyzer-cplusplus.Move].

Motivation

UB is bad.

Context

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This is an automatic backport of pull request #11837 done by [Mergify](https://mergify.com).