Add a "signature" to AST files to verify that they haven't changed

Since the order of the IDs in the AST file (e.g. DeclIDs, SelectorIDs)
is not stable, it is not safe to load an AST file that depends on
another AST file that has been rebuilt since the importer was built,
even if "nothing changed". We previously used size and modtime to check
this, but I've seen cases where a module rebuilt quickly enough to foil
this check and caused very hard to debug build errors.

To save cycles when we're loading the AST, we just generate a random
nonce value and check that it hasn't changed when we load an imported
module, rather than actually hash the whole file.

This is slightly complicated by the fact that we need to verify the
signature inside addModule, since we might otherwise consider that a
mdoule is "OutOfDate" when really it is the importer that is out of
date. I didn't see any regressions in module load time after this
change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220493 91177308-0d34-0410-b5e6-96231b3b80d8
patch to issue warning on comparing parameters with
nonnull attribute when comparison is always
true/false. Patch by Steven Wu with few fixes and minor
refactoring and adding tests by me. rdar://18712242


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220496 91177308-0d34-0410-b5e6-96231b3b80d8
Remove code duplication and cut dependency from clangRewrite on
clangAST.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220502 91177308-0d34-0410-b5e6-96231b3b80d8
Import clang-tools-extra as submodule