[MLIR][OpenMP][OMPIRBuilder] Use target triple to initialize `IsGPU` flag

This patch modifies the construction of the `OpenMPIRBuilder` in MLIR to
initialize the `IsGPU` flag using target triple information passed down from
the Flang frontend. If not present, it will default to `false`.

This replicates the behavior currently implemented in Clang, where the
`CodeGenModule::createOpenMPRuntime()` method creates a different
`CGOpenMPRuntime` instance depending on the target triple, which in turn has an
effect on the `IsGPU` flag of the `OpenMPIRBuilderConfig` object.

Differential Revision: https://reviews.llvm.org/D151903