Merging r367304:

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r367304 | ro | 2019-07-30 12:33:20 +0200 (Tue, 30 Jul 2019) | 10 lines

[CMake] Define _FILE_OFFSET_BITS=64 on Solaris

This is the compantion patch to https://reviews.llvm.org/D64482, needed to ensure
that builds with host compilers that don't yet predefine _FILE_OFFSET_BITS=64 on
Solaris succeed by always making the host and freshly built clang consistent.

Tested on x86_64-pc-solaris2.11.

Differential Revision: https://reviews.llvm.org/D64483

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llvm-svn: 367526