[clang-lit] Added the run_long_tests param option/long_tests feature to toggle...

[clang-lit] Added the run_long_tests param option/long_tests feature to toggle execution of long running FileCheck tests.

This will allow for longer running FileCheck based tests to be committed to
clang for use on buildbots, preventing the normal make-check cycle from
increasing in time significantly.

This is a necessary change in order to commit the end-to-end arm neon intrinsic
tests since FileCheck takes ~20 seconds to run said test due to the large amount
of neon intrinsics.

To force a test to run only when --param run_long_tests=true is passed in use
the following requires statement:

// REQUIRES: long_tests

llvm-svn: 184385