[llvm] fix issue #11916

[yurybura]

Fixes #11916

[vicroms]

Thanks for the PR!

[MVoz]

@yurybura

add -DLLVM_ENABLE_PROJECTS=flang

Installing LLVM Flang Fortran compiler

Since 2015, NVIDIA has supported Flang: a Fortran compiler frontend to LLVM. The first Flang was based on the PGI compiler, with proprietary optimizations removed. However, the C-based decade-plus of cruft led Flang developers and management to a ground-up rewrite in C++17. This new Flang f18 targets modern Fortran 2008 / 2018 syntax and is implemented in C++17 internally. Flang became part of LLVM 11 in April 2020.

NOTE: Flang is growing in features and may not be sufficiently feature-complete for modern Fortran until later LLVM 11.x releases.

Select Flang in the build system as usual by environment variable:
FC=flang
Build Flang
If you it necessary to build Flang, follow the LLVM build procedure with flag -DLLVM_ENABLE_PROJECTS=flang

Some Common options:
    -DLLVM_ENABLE_PROJECTS='...' — semicolon-separated list of the LLVM subprojects you’d like to additionally build. Can include any of: clang, clang-tools-extra, libcxx, libcxxabi, libunwind, lldb, compiler-rt, lld, polly, or debuginfo-tests.
    For example, to build LLVM, Clang, libcxx, and libcxxabi, use -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi".