[msan] Correct @LINE expression in obstack.cc

Summary:

[[@LINE-30]] only worked because the resulting 3 matches the first character of
30. With the additional blank lines the resulting 5 no longer matches 30.

Reviewers: eugenis

Subscribers: eugenis, llvm-commits

Differential Revision: https://reviews.llvm.org/D23515

llvm-svn: 278715