Merging r249854:

------------------------------------------------------------------------
r249854 | kfischer | 2015-10-09 13:24:54 -0400 (Fri, 09 Oct 2015) | 11 lines

Clear SectionSymbols in MCContext::Reset

This was just forgotten when SectionSymbols was introduced and could cause
corruption if the MCContext was reused after Reset.

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13547

------------------------------------------------------------------------

llvm-svn: 252802