Make "@name =" mandatory for globals in .ll files.

An oddity of the .ll syntax is that the "@var = " in

@var = global i32 42

is optional. Writing just

global i32 42

is equivalent to

@0 = global i32 42

This means that there is a pretty big First set at the top level. The
current implementation maintains it manually. I was trying to refactor
it, but then started wondering why keep it a all. I personally find the
above syntax confusing. It looks like something is missing.

This patch removes the feature and simplifies the parser.

llvm-svn: 269096