Add parentheses to silence warning.

DWARFContext.cpp:356:20: error: using the result of an assignment as a condition without parentheses [-Werror,-Wparentheses]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346365 91177308-0d34-0410-b5e6-96231b3b80d8