[typo-correction] Apply name specifier corrections when forming a NNS

Previously we would leave behind the old name specifier prefix, which
creates an invalid AST.  Other callers of CorrectTypo update their
CXXScopeSpec objects with the correction specifier if one is present.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260993 91177308-0d34-0410-b5e6-96231b3b80d8