Rework HeaderSearch's interface for getting a module from a name and

Rework HeaderSearch's interface for getting a module from a name and
for getting the name of the module file, unifying the code for
searching for a module with a given name (into lookupModule()) and
separating out the mapping to a module file (into
getModuleFileName()). No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149197 91177308-0d34-0410-b5e6-96231b3b80d8