Avoid shadowing the global filename. Reorg mangle_filename to be more
precise in the length accounting and use memcpy instead of
strcpy/strcat.

Differential Revision: http://llvm-reviews.chandlerc.com/D2547


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@199332 91177308-0d34-0410-b5e6-96231b3b80d8