Fix how LLVMOPTIONALCOMPONENTS is passed to llvm-build

Patch by Force.Charlie-I

If LLVM_USE_INTEL_JITEVENTS and LLVM_USE_OPROFILE  not set,
"${LLVMOPTIONALCOMPONENTS}" is empty, but
**--enable-optional-components** need arg, Cause
**--write-library-table** to be skipped parsed.

Differential Revision: https://reviews.llvm.org/D47982

llvm-svn: 334543