rts: More const correct-ness fixes

In addition to more const-correctness fixes this patch fixes an
infelicity of the previous const-correctness patch (995cf0f356) which
left `UNTAG_CLOSURE` taking a `const StgClosure` pointer parameter
but returning a non-const pointer. Here we restore the original type
signature of `UNTAG_CLOSURE` and add a new function
`UNTAG_CONST_CLOSURE` which takes and returns a const `StgClosure`
pointer and uses that wherever possible.

Test Plan: Validate on Linux, OS X and Windows

Reviewers: Phyx, hsyl20, bgamari, austin, simonmar, trofi

Reviewed By: simonmar, trofi

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2231

1 files changed, 1 insertions, 1 deletions

diff --git a/rts/Stable.c b/rts/Stable.c
index 2c9480a559..9f34072e61 100644
--- a/rts/Stable.c
+++ b/rts/Stable.c
@@ -377,7 +377,7 @@ StgWord
 lookupStableName (StgPtr p)
 {
   StgWord sn;
-  void* sn_tmp;
+  const void* sn_tmp;
 
   stableLock();