[OPENMP] Make -fopenmp to turn on OpenMP support by default, clang part

[ABataev]

Summary

Patch turns on OpenMP support in clang by default after fixing OpenMP buildbots.

[chandlerc]

LGTM to land when D13802 lands. See my comments there.

cfe/trunk/CMakeLists.txt

 
 set(C_INCLUDE_DIRS "" CACHE STRING
   "Colon separated list of directories clang will search for headers.")
 
 set(GCC_INSTALL_PREFIX "" CACHE PATH "Directory where gcc is installed." )
 set(DEFAULT_SYSROOT "" CACHE PATH
   "Default <path> to all compiler invocations for --sysroot=<path>." )
 
-set(CLANG_DEFAULT_OPENMP_RUNTIME "libgomp" CACHE STRING
+set(CLANG_DEFAULT_OPENMP_RUNTIME "libomp" CACHE STRING
   "Default OpenMP runtime used by -fopenmp.")
 
 set(CLANG_VENDOR "" CACHE STRING
   "Vendor-specific text for showing with version information.")
 
 if( CLANG_VENDOR )
   add_definitions( -DCLANG_VENDOR="${CLANG_VENDOR} " )
 endif()

cfe/trunk/docs/UsersManual.rst

 Clang supports all OpenMP 3.1 directives and clauses.  In addition, some
 features of OpenMP 4.0 are supported.  For example, ``#pragma omp simd``,
 ``#pragma omp for simd``, ``#pragma omp parallel for simd`` directives, extended
 set of atomic constructs, ``proc_bind`` clause for all parallel-based
 directives, ``depend`` clause for ``#pragma omp task`` directive (except for
 array sections), ``#pragma omp cancel`` and ``#pragma omp cancellation point``
 directives, and ``#pragma omp taskgroup`` directive.
 
-OpenMP support is disabled by default. Use :option:`-fopenmp=libomp` to enable
-it. Support for OpenMP can be disabled with :option:`-fno-openmp`.
+Use :option:`-fopenmp` to enable OpenMP. Support for OpenMP can be disabled with
+:option:`-fno-openmp`.
 
 Controlling implementation limits
 ---------------------------------
 
 .. option:: -fopenmp-use-tls
 
  Controls code generation for OpenMP threadprivate variables. In presence of
  this option all threadprivate variables are generated the same way as thread