[MS ABI]: Fix mangling function arguments for template types to be compatible with MSVC

MS name mangling supports cache for first 10 distinct function
arguments.  The error was when non cached template type occurred twice
(e.g. 11th and 12th).  For such case in code there is another cache
table TemplateArgStrings (for performance reasons).  Then one '@'
character at the end of the mangled name taken from this table was
missing.  For other cases the missing '@' character was added in
the call to mangleSourceName(TemplateMangling) in the cache miss code,
but the cache hit code didn't add it.

This fixes a regression from r362560.

Patch by Adam Folwarczny <adamf88@gmail.com>!

Differential Revision: https://reviews.llvm.org/D68099

llvm-svn: 374543