PTH: Replace ad hoc 'file name' -> 'PTH data' lookup table in the PTH file...

PTH: Replace ad hoc 'file name' -> 'PTH data' lookup table in the PTH file with an on-disk chained hash table.  This data structure is implemented using templates, and will be used to replace similar data structures.  This change leads to no visibile performance impact on Cocoa.h, but now we only pay a price for the table on order with the number of files accessed and not the number in the PTH file.

llvm-svn: 64245