[DWARF] Generate qualified names of functions if linkage names are missing.

Summary:
This is similar to the change introduced for variable DIEs in r233098. If the
linkage names of functions are missing in the DWARF, then their fully qualified
names (similar to the name that would be got by demangling their linkage name)
is generated using the decl context.

This change fixes TestNamespace when the test case is compiled with GCC, hence
it is enabled for GCC. The test and the test case are also enhanced to cover
variadic functions.

Test Plan: dotest.py -C <clang|gcc> -p TestNamespace

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8623

llvm-svn: 233336