Add findFunctionInfoList() accessor to FunctionInfoIndex.

Summary:
This allows to query for a function in the map without creating an
entry, allowing to use a const FunctionInfoIndex.

Reviewers: tejohnson

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14912

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 253953