Merging r309555:

Merging r309555:
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r309555 | mstorsjo | 2017-07-31 04:18:41 -0700 (Mon, 31 Jul 2017) | 10 lines

[llvm-dlltool] Write correct weak externals

Previously, the created object files for the import library were broken.
Write the symbol table before the string table. Simplify the code by
using a separate variable Prefix instead of duplicating a few lines.

Also update the coff-weak-exports to actually check that the generated
weak symbols can be found as intended.

Differential Revision: https://reviews.llvm.org/D36065
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_50@309837 91177308-0d34-0410-b5e6-96231b3b80d8