Merging r347431:

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r347431 | rnk | 2018-11-21 14:01:10 -0800 (Wed, 21 Nov 2018) | 12 lines

[mingw] Use unmangled name after the $ in the section name

GCC does it this way, and we have to be consistent. This includes
stdcall and fastcall functions with suffixes. I confirmed that a
fastcall function named "foo" ends up in ".text$foo", not
".text$@foo@8".

Based on a patch by Andrew Yohn!

Fixes PR39218.

Differential Revision: https://reviews.llvm.org/D54762
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llvm-svn: 347931