Discard uncompressed buffer after creating .gdb_index contents.

Once we create .gdb_index contents, .zdebug_gnu_pub{names,types}
are useless, so there's no need to keep their uncompressed data
in memory.

I observed that for a test case in which lld creates a 3GB .gdb_index
section, the maximum resident set size reduced from 43GB to 29GB after
this patch.

Differential Revision: https://reviews.llvm.org/D52126

llvm-svn: 342297