[BPF] do not generate unused local/global types

[BPF] do not generate unused local/global types

The kernel currently has a limit for # of types to be 64KB and
the size of string subsection to be 64KB. A simple bcc tool
runqlat.py generates:
  . the size of ~33KB type section, roughly ~10K types
  . the size of ~17KB string section

The majority type is from the types referenced by local
variables in the bpf program. For example, the kernel "task_struct"
itself recursively brings in ~900 other types.
This patch did the following optimization to avoid generating
unused types:
  . do not generate types for local variables unless they are
    function arguments.
  . do not generate types for external globals.

If an external global is not used in the program, llvm
already removes it from IR, so global variable saving is
typical small. For runqlat.py, only one variable "llvm.used"
is the external global.

The types for locals and external globals can be added back
once there is a usage for them.

After the above optimization, the runqlat.py generates:
  . the size of ~1.5KB type section, roughtly 500 types
  . the size of ~0.7KB string section

UPDATE:
  resubmitted the patch after previous revert with
  the following fix:
  use Global.hasExternalLinkage() to test "external"
  linkage instead of using Global.getInitializer(),
  which will assert on external variables.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 356234