MCDwarf: Simplify MCDwarfFile to just use std::string instead of cunning use of MCContext's allocator.

There aren't /that/ many files, and we are already using various maps
and other standard containers that don't use MCContext's allocator to
store these values, so this doesn't seem to be critical and simplifies
the design (I'll be moving construction out of MCContext shortly so it'd
be annoying to have to pass the allocator around to allocate these
things... and we'll have non-MCContext users (debug_line.dwo) shortly)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203831 91177308-0d34-0410-b5e6-96231b3b80d8