fix a compilation error with latest llvm/clang trunk

bcc build with latest llvm/clang trunk failed with the
below error:
......
[ 35%] Built target api-static
/home/yhs/work/bcc/src/cc/frontends/clang/tp_frontend_action.cc: In member function ‘bool ebpf::TracepointTypeVisitor::Visit
FunctionDecl(clang::FunctionDecl*)’:
/home/yhs/work/bcc/src/cc/frontends/clang/tp_frontend_action.cc:163:60: error: no matching function for call to ‘clang::Qual
Type::getAsString(clang::SplitQualType)’
         auto type_name = QualType::getAsString(type.split());
                                                            ^
......

The error is caused by the below clang commit:
commit fcc28fd8cc8139cf1e4763459447880768579d8e
Author: Aaron Ballman <aaron@aaronballman.com>
Date:   Thu Dec 21 21:42:42 2017 +0000

    Re-commit r321223, which adds a printing policy to the ASTDumper.
......
-  std::string getAsString() const {
-    return getAsString(split());
+  static std::string getAsString(SplitQualType split,
+                                 const PrintingPolicy &Policy) {
+    return getAsString(split.Ty, split.Quals, Policy);
   }
+  static std::string getAsString(const Type *ty, Qualifiers qs,
+                                 const PrintingPolicy &Policy);

-  static std::string getAsString(SplitQualType split) {
-    return getAsString(split.Ty, split.Quals);
-  }
-
-  static std::string getAsString(const Type *ty, Qualifiers qs);
-
+  std::string getAsString() const;
   std::string getAsString(const PrintingPolicy &Policy) const;
......

The signature of static function getAsString(), which is used
in src/cc/frontends/clang/tp_frontend_action.cc, got changed,
and this caused the compilation error.

The patch chooses a different way to get type_name which works
for llvm 4.0 to 6.0 (tested).
Signed-off-by: Yonghong Song <yhs@fb.com>

CentOS 6 support

fix build issue for llvm 5.0.1

Fix issue #1502.
A few cmake version checking greater than 5 includes 5.0.1
which results in compilation failure. Change version checking
to explicitly equal to or greater than version 6.
Signed-off-by: Yonghong Song <yhs@fb.com>

ucalls: convert bytes to str

Fix incorrect lost_cb type in Python

Fix and improvement for perf_reader's lost_cb

Handle unknown user in cachetop

tcpretrans: extend for sum up

histogram: support sorting bucket before dumping table

Improve trace.py formatting and printing options

add a test case for bucket_sort_fn and show its
use scenario as well

add examples for including additional header files

Update for new bpf_perf_{event|prog}_read_value helpers

Clean-up unused parameter in tracing APIs

cmake3 on Ubuntu Trusty

- Set `inherit = 1` only for per-task perf event. Inspired from change
  https://github.com/torvalds/linux/commit/81b9cf8028a17bdbdaa0da80b735b32150d4e89e
- Kernel now support more events (see #1448). Also it's checked in
  `bpf_attach_perf_event` already. Remove the check from C++ API

This fixes the following error with python3

Tracing calls in process 2577 (language: python)... Ctrl-C to quit.
^C
Traceback (most recent call last):
  File "/usr/share/bcc/tools/lib/ucalls", line 303, in <module>
    data = get_data()   # [(function, (num calls, latency in ns))]
  File "/usr/share/bcc/tools/lib/ucalls", line 264, in get_data
    bpf["counts"].items()))
  File "/usr/share/bcc/tools/lib/ucalls", line 262, in <lambda>
    data = list(map(lambda kv: (kv[0].clazz + "." + kv[0].method,
TypeError: can't concat str to bytes
Signed-off-by: Rafael Fonseca <r4f4rfs@gmail.com>

Add C++ helper to clear BPF tables

Feature to quickly discern heavily retransmitting flows and
therefore spotting culprit network paths.
Signed-off-by: Matthias Tafelmeier <matthias.tafelmeier@gmx.net>

Fix issue #1478
Two tools, trace.py and argdist.py, and their corresponding
example files were changed.
Fixed a minor typo in one of trace.py error messages.
Signed-off-by: Yonghong Song <yhs@fb.com>

add padding to the end of structure