[OpenMP] Fix LLVM_ENABLE_PROJECTS=openmp build (#151117)

Set LLVM_TREE_AVAILABLE when not defined after #149871. In particular,
the LLVM build tree is obviously available with
`add_subdirectory(openmp)` from the LLVM build tree itself. Note that
this build mode is deprecated since #136314.

GitOrigin-RevId: 9b2b5bf8cd751fabccaca1b8847b4c291aa52081
1 file changed