perf tools: Fix LLVM test failure when running in verbose mode

A CI system might want to run all tests in verbose mode so that there is
enough information to diagnose issues. This LLVM test is the only test
that uses "-v" to signify to not skip the test if the preconditions
aren't met (LLVM isn't installed). This means that running the test in
verbose mode without LLVM installed causes a test failure.

For consistency with the other tests, remove this verbose/skip check. An
alternate solution would be to make _all_ tests not skip when run in
verbose mode, but I don't think that would be intuitive.

Also change the search_program() call to search_program_and_warn().
Previously the hint about installing LLVM was only printed by the actual
test because this check was skipped in verbose mode. To maintain the old
behaviour, the precondition check must also print the full warning.

Previous output:

  $ ./perf test llvm
  40: LLVM search and compile                                     :
  40.1: Basic BPF llvm compile                                    : Skip

  $ ./perf test -v llvm
  40: LLVM search and compile                                     :
  40.1: Basic BPF llvm compile                                    :
  --- start ---
  test child forked, pid 2085835
  ERROR:	unable to find clang.
  Hint:	Try to install latest clang/llvm to support BPF. Check your $PATH
  ...
  test child finished with -1
  ---- end ----
  LLVM search and compile subtest 1: FAILED!

New output (non verbose mode is identical, verbose changes from fail to
skip):

  $ ./perf test llvm
  40: LLVM search and compile                                     :
  40.1: Basic BPF llvm compile                                    : Skip

  $ ./perf test -v llvm
  40: LLVM search and compile                                     :
  40.1: Basic BPF llvm compile                                    :
  --- start ---
  test child forked, pid 2087680
  ERROR:	unable to find clang.
  Hint:	Try to install latest clang/llvm to support BPF. Check your $PATH
  ...
  No clang, skip this test
  test child finished with -2
  ---- end ----
  LLVM search and compile subtest 1: Skip

Signed-off-by: James Clark <james.clark@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: clang-built-linux@googlegroups.com
Link: http://lore.kernel.org/lkml/20210831145501.2135754-2-james.clark@arm.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>