Simplify the logic for emitting guard variables for template static

Simplify the logic for emitting guard variables for template static
data members by delaying the emission of the initializer until after
linkage and visibility have been set on the global.  Also, don't
emit a guard unless the variable actually ends up with vague linkage,
and don't use thread-safe statics in any case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@118336 91177308-0d34-0410-b5e6-96231b3b80d8