[OpenMP] Remove dependency on LLVM include directory from DeviceRTL (#136359)
Summary:
Currently we depend on a single LLVM include directory. This is actually
only required to define one enum, which is highly unlikely to change.
THis patch makes the `Environment.h` include directory more hermetic so
we no long depend on other libraries. In exchange, we get a simpler
dependency list for the price of hard-coding `1` somewhere. I think it's
a valid trade considering that this flag is highly unlikely to change at
this point.
@ronlieb AMD version
https://gist.github.com/jhuber6/3313e6f957be14dc79fe85e5126d2cb3