[globalopt] Change so that emitting fragments doesn't use the type size of DIVariables

When turning on -debug-info-kind=constructor we ran into a "fragment covers
entire variable" error during thinlto. The fragment is currently always
emitted if there is no type size, but sometimes the variable has a
forward declared struct type which doesn't have a size.

This changes the code to get the type size from the GlobalVariable instead.

Differential Revision: https://reviews.llvm.org/D85572