Enable -Wconversion for GHC's C code

Because apparently none of -Wall, -Wextra or -pedantic of GCC imply -Wconversion or its subset -Wsign-conversion: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51625

That means currently you can call a function taking an unsigned int with a signed int without getting any warning.

That is very bad, because of course negative numbers will overflow.

Having -Wconversion would have caught overflow errors such as this one: https://phabricator.haskell.org/D3955#110993

We should probably have it for the C code.

Turning it on everywhere will be quite some work though because it creates lots of complaints such as long long int not being the same as long int (which is technically correct but in practice they are typically the same).

Trac metadata

Trac field	Value
Version	8.2.1
Type	Task
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	bgamari, nh2
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Architecture