Modules: set SystemHeader to true if we are building a system module.

If we are processing a #include from a module build, we should treat it
as a system header if we're building a system module. Passing an optional
flag to HeaderSearch::LookupFile.

Before this, the testing case will crash when accessing a freed FileEntry.

rdar://26214027


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269730 91177308-0d34-0410-b5e6-96231b3b80d8