[include-fixer] Simplify unittest code.

Summary:
The InMemorySymbolIndex only constructs hard-coded Class symbols from a string
map. The patch provide a flexable way in InMemorySymbolIndex to create a custom
SymbolInfo in the include-fixer unittest.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20232

llvm-svn: 269427