CmmToC: don't add extern decl to parsed Cmm data

Previously, if a .cmm file *not in the RTS* contained something like:

```cmm
section "rodata" { msg : bits8[] "Test\n"; }
```

It would get compiled by CmmToC into:

```c
ERW_(msg);
const char msg[] = "Test\012";
```

and fail with:

```
/tmp/ghc32129_0/ghc_4.hc:5:12: error:
     error: conflicting types for \u2018msg\u2019
     const char msg[] = "Test\012";
                ^~~

In file included from /tmp/ghc32129_0/ghc_4.hc:3:0: error:

/tmp/ghc32129_0/ghc_4.hc:4:6: error:
     note: previous declaration of \u2018msg\u2019 was here
     ERW_(msg);
          ^

/builds/hsyl20/ghc/_build/install/lib/ghc-8.11.0.20200605/lib/../lib/x86_64-linux-ghc-8.11.0.20200605/rts-1.0/include/Stg.h:253:46: error:
     note: in definition of macro \u2018ERW_\u2019
     #define ERW_(X)   extern       StgWordArray (X)
                                                  ^
```

See the rationale for this on https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/compiler/backends/ppr-c#prototypes

Now we don't generate these extern declarations (ERW_, etc.) for
top-level data. It shouldn't change anything for the RTS (the only place
we use .cmm files) as it is already special cased in
`GHC.Cmm.CLabel.needsCDecl`. And hand-written Cmm can use explicit
extern declarations when needed.

Note that it allows `cgrun069` test to pass with CmmToC (cf #15467).

1 files changed, 1 insertions, 2 deletions

diff --git a/testsuite/tests/codeGen/should_run/all.T b/testsuite/tests/codeGen/should_run/all.T
index 5db0dcff74..4a70914bb0 100644
--- a/testsuite/tests/codeGen/should_run/all.T
+++ b/testsuite/tests/codeGen/should_run/all.T
@@ -75,8 +75,7 @@ test('cgrun066', normal, compile_and_run, [''])
 test('cgrun067', [extra_files(['Cgrun067A.hs'])], compile_and_run, [''])
 test('cgrun068', reqlib('random'), compile_and_run, [''])
 test('cgrun069',
-     [when(unregisterised(), expect_broken(15467)),
-      omit_ways(['ghci'])],
+     [ omit_ways(['ghci'])],
      multi_compile_and_run,
      ['cgrun069', [('cgrun069_cmm.cmm', '')], ''])
 test('cgrun070', normal, compile_and_run, [''])