make the redeclaration case faster for the common instance of a redeclaration
where there is exactly one existing declaration.  This is common.

this speeds up clang about 3% on cocoa.h for me 0.165 -> 0.160s


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@65096 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed