[mips] Make TTypeEncoding indirect to allow .eh_frame to be read-only.

Summary:
Following on from r209907 which made personality encodings indirect, do the
same for TType encodings. This fixes the case where a try/catch block needs
to generate references to, for example, std::exception in the
.gcc_except_table.

Reviewers: petarj

Reviewed By: petarj

Subscribers: srhines, joerg, tberghammer, llvm-commits

Differential Revision: http://reviews.llvm.org/D9669

llvm-svn: 238427