encode the type and decl offsets with 32-bits for entry instead

of 64 bits.  This cuts 400KB off the PCH file for cocoa (7.1 -> 
6.7MB):

Before:

	Record Histogram:
		  Count    # Bits   % Abv  Record Kind
		      1     14296          SOURCE_LOCATION_PRELOADS
		      1   1699598  100.00  SOURCE_LOCATION_OFFSETS
		      1   1870766  100.00  METHOD_POOL
		      1    212988  100.00  SELECTOR_OFFSETS
		      1        88          STATISTICS
		      1       106          SPECIAL_TYPES
		      1  18033788  100.00  IDENTIFIER_TABLE
		      1   1806428  100.00  IDENTIFIER_OFFSET
		      1       170  100.00  TARGET_TRIPLE
		      1       268          LANGUAGE_OPTIONS
		      1   5168252  100.00  DECL_OFFSET
		      1    952700  100.00  TYPE_OFFSET

After:

	Record Histogram:
		  Count    # Bits   % Abv  Record Kind
		      1     14296          SOURCE_LOCATION_PRELOADS
		      1   1699598  100.00  SOURCE_LOCATION_OFFSETS
		      1   1870766  100.00  METHOD_POOL
		      1    212988  100.00  SELECTOR_OFFSETS
		      1        88          STATISTICS
		      1       106          SPECIAL_TYPES
		      1  18033788  100.00  IDENTIFIER_TABLE
		      1   1806428  100.00  IDENTIFIER_OFFSET
		      1       170  100.00  TARGET_TRIPLE
		      1       268          LANGUAGE_OPTIONS
		      1   2584156  100.00  DECL_OFFSET
		      1    476380  100.00  TYPE_OFFSET

llvm-svn: 70216