Remove append parameter to FindGlobalVariables

Summary:
As discussed in https://bugs.llvm.org/show_bug.cgi?id=37317,
FindGlobalVariables does not properly handle the case where
append=false.  As this doesn't seem to be used in the tree, this patch
removes the parameter entirely.

Reviewers: clayborg, jingham, labath

Reviewed By: clayborg

Subscribers: aprantl, lldb-commits, kubamracek, JDevlieghere

Differential Revision: https://reviews.llvm.org/D46885
Patch by Tom Tromey <ttromey@mozilla.com>.

llvm-svn: 333639