Merging r339170:

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r339170 | mstorsjo | 2018-08-07 22:02:40 +0200 (Tue, 07 Aug 2018) | 5 lines

[Headers] Expand _Unwind_Exception for SEH on MinGW/x86_64

This matches how GCC defines this struct.

Differential Revision: https://reviews.llvm.org/D50380
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_70@339220 91177308-0d34-0410-b5e6-96231b3b80d8