Merging r353463:

Merging r353463:
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r353463 | smeenai | 2019-02-07 21:58:04 +0100 (Thu, 07 Feb 2019) | 4 lines

[cmake] Pass LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN to NATIVE configure

We should propagate this down to host builds so that e.g. people using
an optimized tablegen can do the sub-configure successfully.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_80@353517 91177308-0d34-0410-b5e6-96231b3b80d8