Fix the merging of the constantness of declarations.

The langref says:

LLVM explicitly allows declarations of global variables to be marked
constant, even if the final definition of the global is not. This
capability can be used to enable slightly better optimization of the
program, but requires the language definition to guarantee that
optimizations based on the ‘constantness’ are valid for the
translation units that do not include the definition.

Given that definition, when merging two declarations, we have to drop
constantness if of of them is not marked contant, since the Module
without the constant marker might not have the necessary guarantees.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220927 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Don't manually parse an option for no reason

We're using cl::opt here, but for some reason we're reading out one
particular option by hand instead. This makes -help and the like
behave rather poorly, so let's not do it this way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220928 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Follow LLVM naming conventions

This renames a few things that are using an unusual naming convention.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220929 91177308-0d34-0410-b5e6-96231b3b80d8