Fix lock order inversion between ManagedStatic and Statistic

Summary:
Statistic and ManagedStatic both use mutexes. There was a lock order
inversion where, during initialization, Statistic's mutex would be
held while taking ManagedStatic's, and in llvm_shutdown,
ManagedStatic's mutex would be held while taking Statistic's
mutex. This change causes Statistic's initialization code to avoid
holding its mutex while calling ManagedStatic's methods, avoiding the
inversion.

Reviewers: dsanders, rtereshin

Reviewed By: dsanders

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D45398

llvm-svn: 330236