When a header is specified via the -File=... or -FileList=... arguments, we scan the engine and project directories for source files and parse the include directives from them. A lookup table is built that maps from filename to files that include it, allowing building a (conservative) superset of the files which may include it.
#preflight 61bd194a6041819fd179bf21
[CL 18486253 by Ben Marsh in ue5-main branch]
