Merge 110141b37813dc48af33de5e1407231e56acdfc5 into 18.x

[brad0]

[OpenMP] fix endianness dependent definitions in OMP headers
for MSVC (#84540)

MSVC does not define BYTE_ORDER making the check for BigEndian
erroneously evaluate to true and breaking the struct definitions in MSVC
compiled builds correspondingly. The fix adds an additional check for
whether BYTE_ORDER is defined by the compiler to fix these.

[brad0]

/cherry-pick 110141b

[llvmbot]

@llvm/issue-subscribers-openmp

Author: Brad Smith (brad0)

[OpenMP] fix endianness dependent definitions in OMP headers for MSVC (#84540)

MSVC does not define BYTE_ORDER making the check for BigEndian
erroneously evaluate to true and breaking the struct definitions in MSVC
compiled builds correspondingly. The fix adds an additional check for
whether BYTE_ORDER is defined by the compiler to fix these.

[llvmbot]

/pull-request #84668