Resolve modulemaps after the HeaderSearch is initialized.
Resolve modulemaps after the HeaderSearch is initialized.

This patch allows us to resolve system-specific header search directories
such as the location of libc.

This patch fixes experimental runtime_cxxmodules on OSX where libc is in
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/

root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling