Fix a miscompile introduced in r220178.

The original code had an implicit assumption that if the test for
allocas or globals was reached, the two pointers were not equal. With my
changes to make the pointer analysis more powerful here, I also had to
guard against circumstances where the results weren't useful. That in
turn violated the assumption and gave rise to a circumstance in which we
could have a store with both the queried pointer and stored pointer
rooted at *the same* alloca. Clearly, we cannot ignore such a store.
There are other things we might do in this code to better handle the
case of both pointers ending up at the same alloca or global, but it
seems best to at least make the test explicit in what it intends to
check.

I've added tests for both the alloca and global case here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220190 91177308-0d34-0410-b5e6-96231b3b80d8
Clean up the comments and doxygen for DataLayout.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220191 91177308-0d34-0410-b5e6-96231b3b80d8
Remove some completely superfluous trailing comments and clang-format
this header to remove numerous formatting inconsistencies that impede
making simple changes here without large diffs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220192 91177308-0d34-0410-b5e6-96231b3b80d8
Switch the default DataLayout to be little endian, and make the variable
be BigEndian so the default can continue to be zero-initialized.

This is one of the prerequisites to making DataLayout a constant and
always available part of every module.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220193 91177308-0d34-0410-b5e6-96231b3b80d8