lld-link: Separate 'undefined symbol' errors with just one newline, not two.

newline() in ErrorHandler.cpp already tries to insert newlines between messages
that contain embedded newlines, so getSymbolLocations() shouldn't return a
string that ends in a newline -- else we end up with two newlines between error
messages.

Makes lld-link's output look more like ld.lld output.

https://reviews.llvm.org/D51117

llvm-svn: 340482