Alternative to https://reviews.llvm.org/D68101
Prevents globals with an explicit section from being mergeable by default (as in D68101).
Adds a command line option to allow the explicit section to be mergeable; but, an error will be emitted if broken output would be created.
Users can use the command line option if the default behaviour results in a performance regression.
This is a partial fix for https://bugs.llvm.org/show_bug.cgi?id=43457.