OpenMP `no_alloc` map type modifier extension

[jdenny-ornl]

Clacc prototypes a no_alloc map type modifier as an OpenMP extension to implement the OpenACC no_create clause. The initial commit is 2a55d45. This feature should be considered for upstreaming to LLVM as described in #10.

See the sections "Command-Line Options" and "no_create Clause's Presence Check" in Clacc's clang/README-OpenACC-status.md. See the section "OpenACC to OpenMP Mapping" in Clacc's clang/README-OpenACC-design.md.