to increased calls to SourceManager::getFileID. (rdar://9992664)

Use a slightly different approach that is more efficient both in terms of speed
(no extra getFileID calls) and in SLocEntries reduction.

Comparing pre-r138129 and this patch we get:

For compiling SemaExpr.cpp reduction of SLocEntries by 26%.
For the boost enum library:
  -SLocEntries -34% (note that this was -5% for r138129)
  -Memory consumption -50%
  -PCH size -31%

Reduced SLocEntries also benefit the hot function SourceManager::getFileID,
evident by the reduced "FileID scans".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138380 91177308-0d34-0410-b5e6-96231b3b80d8