Add a new version of Module::getFunction that takes a const char* instead

Add a new version of Module::getFunction that takes a const char* instead
of a std::string.  This avoids copying the string to the heap in common
cases.  Patch by Pratik Solanki!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52834 91177308-0d34-0410-b5e6-96231b3b80d8