[LLD/PDB] Write actual records to the globals stream.

Previously we were writing an empty globals stream.  Windows
tools interpret this as "private symbols are not present in
this PDB", even when they are, so we need to fix this.  Regardless,
without it we don't have information about global variables, so
we need to fix it anyway.  This patch does that.

With this patch, the "lm" command in WinDbg correctly reports
that we have private symbols available, but the "dv" command
still refuses to display local variables.

Differential Revision: https://reviews.llvm.org/D36535

llvm-svn: 310743