perf clang: Do not use 'return std::move(something)'

perf clang: Do not use 'return std::move(something)'

It prevents copy elision, generating this warning when building with
fedora:rawhide's clang:

  clang version 7.0.1 (Fedora 7.0.1-2.fc30)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix
  InstalledDir: /usr/bin
  Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/9
  Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/9
  Selected GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/9
  Candidate multilib: .;@m64
  Candidate multilib: 32;@m32
  Selected multilib: .;@m64

  $ make -C tools/perf CC=clang LIBCLANGLLVM=1
  <SNIP>
  util/c++/clang.cpp: In function 'std::unique_ptr<llvm::SmallVectorImpl<char> > perf::getBPFObjectFromModule(llvm::Module*)':
  util/c++/clang.cpp:163:18: error: moving a local object in a return statement prevents copy elision [-Werror=pessimizing-move]
    163 |  return std::move(Buffer);
        |         ~~~~~~~~~^~~~~~~~
  util/c++/clang.cpp:163:18: note: remove 'std::move' call
  cc1plus: all warnings being treated as errors
  <SNIP>

References:

  http://www.cplusplus.com/forum/general/186411/#msg908572
  https://en.cppreference.com/w/cpp/language/return#Notes
  https://en.cppreference.com/w/cpp/language/copy_elision

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-lehqf5x5q96l0o8myhb6blz6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>