Add a hook to supply a custom CompilationDatabase. To add a custom CompilationDatabase, make it implement findCompilationDatabaseForDirectory in CustomCompilationDatabase.h and set USE_COSTUM_COMPILATION_DATABASE.

Differential Revision: http://llvm-reviews.chandlerc.com/D4

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@160061 91177308-0d34-0410-b5e6-96231b3b80d8