Change llvm call once check for building Swift for PowerPC(ppc64le)

[sarveshtamba]

Change llvm call once check for building Swift for PowerPC(ppc64le). These changes are required to resolve the call_once related errors seen while building the Swift toolchain on PowerPC64LE.

[sarveshtamba]

This code change needs to go in along with following PRs:
swiftlang/swift#21038
swiftlang/swift-package-manager#1906

[bob-wilson]

Can you get this into the upstream LLVM sources first? This change does not seem like it is specific to Swift.

[sarveshtamba]

Bug 39912 has been successfully created.
Also I have posted this patch on LLVM's Phabricator suggestion. Following is the link for patch review:-
https://reviews.llvm.org/D55503
Please let me know if anything else is required.

[bob-wilson]

Can you change this PR to be a cherry-pick of r348970 (304fa37)?

[sarveshtamba]

Hi Bob, please see my comments in https://reviews.llvm.org/D55503

[bob-wilson]

Yes, I saw that. I agree that we'll need to figure out the right solution on trunk before cherry-picking anything here.

[compnerd]

I'm going to close this PR. The necessary changes have been made in LLVM upstream. If they aren't in the 5.1 branch, we can pull them into the 5.1 branch as backports.