Fix header-search problems with precompiled headers, where the

presence or absence of header map arguments when using the precompiled
header would cause Clang to get confused about which headers had
already been included/imported, along with their controlling
macros. The fundamental problem is that the serialization of the
header search information was relying on the UIDs of FileEntry objects
at PCH generation time and PCH load time to be equivalent, which
effectively means that we had to probe the same files in the same
order. Differing header map arguments caused an extra FileEntry
lookup, but it's easy to imagine other minor command-line arguments
triggering this problem.

Header-search information is now encoded along with the
source-location entry for a file, so that we register information
about a file's properties as a header at the same time we create the
FileEntry for that file.

Fixes <rdar://problem/7743243>.

llvm-svn: 98636