Make llvm::ConvertDebugDeclareToDebugValue() be a void function (NFC)

The routines llvm::ConvertDebugDeclareToDebugValue() always returned
a true value which was never checked at the call site; change the
function return type to void.

This NFC cleanup was approved in the review https://reviews.llvm.org/D23715

llvm-svn: 281964