Enable a test in TestSTL for GCC.

Summary:
Remove @skipIfGcc from a test in TestSTL as that test passes with GCC
now. Also, mark one of the dsym tests with @skipUnlessDarwin in TestSTL.

Test Plan: dotest.py -C gcc -p TestSTL

Reviewers: vharron

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8793

llvm-svn: 234243