[AIX][TLS] Relax front end diagnostics to accept the local-exec TLS model

This patch relaxes the front end AIX diagnostics added in D102070 to accept the
local-exec TLS model, as we plan to support this model in a series of future patches.

The diagnostics are relaxed when local-exec is used as a compiler option to
`-ftls-model=*` and in the `__attribute__((tls_model("local-exec")))` attribute.

Differential Revision: https://reviews.llvm.org/D149596