-Wshadow does not warn for `static` variables

[alexkaratarakis]

Min repro:

// Compile with -Wshadow
int main()
{
    static bool CONDITION = false;

    {
        static bool CONDITION = true;
    }
}

expected results: declaration of 'CONDITION' shadows a previous local [-Wshadow]
actual results: No warnings

gcc warns when the diagnostic is enabled, but clang does not.

Demo: https://godbolt.org/z/WjPTMvYP8

[hazohelet]

Proposed fix: https://reviews.llvm.org/D151214