Merging r197409:

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r197409 | rikka | 2013-12-16 11:19:18 -0800 (Mon, 16 Dec 2013) | 4 lines

Make Sema::BuildCXXNestedNameSpecifier correctly clear the previous
CXXScopeSpec when necessary while performing typo correction. This fixes
the crash reported in PR18213 (the problem existed since r185487, and
r193020 made it easier to hit).
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llvm-svn: 197463