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Add option to use older clang ABI behavior when passing certain union types as…

Authored by dyung on Oct 19 2020, 6:17 PM.

Description

Add option to use older clang ABI behavior when passing certain union types as function arguments

Recently commit D78699 (commit 26cfb6e562f1), fixed clang's behavior with respect
to passing a union type through a register to correctly follow the ABI. However,
this is an ABI breaking change with earlier versions of the clang compiler, so we
should add an -fclang-abi-compat option to address this. Additionally, the PS4 ABI
requires the older behavior, so that is added as well.

This change adds a Ver11 value to the ClangABI enum that when it is set (or the
target is the PS4 triple), we skip the ABI fix introduced in D78699.

Differential Revision: https://reviews.llvm.org/D89747

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