Fix UBSan bots by blacklisting bits/stl_tree.h.

Summary:
libstdc++ has some undefined behavior in bits/stl_tree.h that
has recently became excercised by some of the LLVM code.
Given that fixing libstdc++ will take years, adding the file
into a blacklist to fix bots seems like a necessity.

Reviewers: vitalybuka

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28686

llvm-svn: 291918