Produce a single string table in a ELF .o

Normally an ELF .o has two string tables, one for symbols, one for section
names.

With the scheme of naming sections like ".text.foo" where foo is a symbol,
there is a big potential saving in using a single one.

Building llvm+clang+lld with master and with this patch the results were:

master:                          193,267,008 bytes
patch:                           186,107,952 bytes
master non unique section names: 183,260,192 bytes
patch non unique section names:  183,118,632 bytes

So using non usique saves 10,006,816 bytes, and the patch saves 7,159,056 while
still using distinct names for the sections.

llvm-svn: 238073