Avoid dsymutil calls to getFileNameByIndex.
This change adds a hasFileAtIndex method. getChildDeclContext can first call this method, and if it returns true it knows it can then lookup the resolved path cache for the given file index. If we hit that cache then we don't even have to call getFileNameByIndex.
Running dsymutil against the swift executable built from github gives a 20% performance improvement without any change in the binary.
Differential Revision: https://reviews.llvm.org/D22655
Reviewed by friss.