Without this change the problem is that flangOmpReport and
flangPrintFunctionNames libraries are not built under 'all',
but they are imported targets via LLVMExports.cmake so that
any out-of-tree build that configures upon LLVM+Flang package
will get this CMake error:
```
The imported target "flangPrintFunctionNames" references the file
".../lib/flangPrintFunctionNames.so"
but this file does not exist.
```
flang-aarch64-out-of-tree buildbot (https://lab.llvm.org/buildbot/#/builders/175)
does not catch this issue, because it does not enable Flang on the first stage.
This change gets rid of FLANG_BUILD_EXAMPLES in favor of LLVM_BUILD_EXAMPLES
and uses available LLVM CMake macros to add example executables/libraries.
Differential Revision: https://reviews.llvm.org/D145992
1c5d121440
