[cxxmodules] Resolve modulemaps after the HeaderSearch is initialized.
[cxxmodules] 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/

olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,olifre/root,karies/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,karies/root