ELF2: Manage BumpPtrAllocator ownership better.

Previously, each ArgParser owned a BumpPtrAllocator, and arguments parsed
by an ArgParser would refer strings allocated using the BumpPtrAllocator
only when response files were used. This could cause a subtle bug because
such ownership was not obvious.

This patch moves the ownership from ArgParser to Driver and make the
ownership explicit.

llvm-svn: 249963