Implement the reader of the global module index and wire it into the
AST reader.

The global module index tracks all of the identifiers known to a set
of module files. Lookup of those identifiers looks first in the global
module index, which returns the set of module files in which that
identifier can be found. The AST reader only needs to look into those
module files and any module files not known to the global index (e.g.,
because they were (re)built after the global index), reducing the
number of on-disk hash tables to visit. For an example source I'm
looking at, we go from 237844 total identifier lookups into on-disk
hash tables down to 126817.

Unfortunately, this does not translate into a performance advantage.
At best, it's a wash once the global module index has been built, but
that's ignore the cost of building the global module index (which
is itself fairly large). Profiles show that the global module index
code is far less efficient than it should be; optimizing it might give
enough of an advantage to justify its continued inclusion.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173405 91177308-0d34-0410-b5e6-96231b3b80d8
Serialization/GlobalModuleIndex.cpp: Fixup r173405, <cstdio>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173408 91177308-0d34-0410-b5e6-96231b3b80d8
[Lex] Remove DirectoryLookup.UserSpecified, which is unused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173409 91177308-0d34-0410-b5e6-96231b3b80d8
[Frontend] The -iwithprefix option belongs in the After category, according to
GCC docs.
 - Found by inspection.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173410 91177308-0d34-0410-b5e6-96231b3b80d8
[Frontend] Drop the isUserSupplied argument to InitHeaderSearch, it is unused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173411 91177308-0d34-0410-b5e6-96231b3b80d8
[Frontend] Remove another IsUserSpecified member variable that is now unused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173412 91177308-0d34-0410-b5e6-96231b3b80d8