Don't segfault in EmitCXXGlobalInitFunc when main file is a membuf

Summary: When the main file is created from a membuffer, there is no file entry that can be retrieved. This uses "__GLOBAL_I_a" in that case which is what was always used before r208128.

Reviewers: majnemer, thakis

Reviewed By: thakis

Subscribers: yaron.keren, rsmith, cfe-commits

Differential Revision: http://reviews.llvm.org/D5043