[Modules] Resolve top-headers of modules lazily.

This allows resolving top-header filenames of modules to FileEntries when
we need them, not eagerly.

Note that that this breaks ABI for libclang functions
clang_Module_getTopLevelHeader / clang_Module_getNumTopLevelHeaders
but this is fine because they are experimental and not widely used yet.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176975 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed