CFI: blacklist STL allocate() from unrelated-casts

Summary:
Previously, STL allocators were blacklisted in compiler_rt's
cfi_blacklist.txt because they mandated a cast from void* to T* before
object initialization completed. This change moves that logic into the
front end because C++ name mangling supports a substitution compression
mechanism for symbols that makes it difficult to blacklist the mangled
symbol for allocate() using a regular expression.

Motivated by crbug.com/751385.

Reviewers: pcc, kcc

Reviewed By: pcc

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36294

llvm-svn: 310097