Sylvain Henry authored Feb 08, 2021 and Marge Bot committed Mar 26, 2021

As suggested by @alexbiehl, this patch replaces the always updated
UniqSupply in NameCache with a fixed Char and use it with `uniqFromMask`
to generate uniques.

This required some refactoring because getting a new unique from the
NameCache can't be done in pure code anymore, in particular not in an
atomic update function for `atomicModifyIORef`. So we use an MVar
instead to store the OrigNameCache field.

For some reason, T12545 increases (+1%) on i386 while it decreases on
other CI runners.

T9630 ghc/peak increases only with the dwarf build on CI (+16%).

Metric Decrease:
    T12425
    T12545
    T9198
    T12234

Metric Increase:
    T12545
    T9630

Update haddock submodule