[PR26550] Use a different TBAA root for C++ vs C.

[PR26550] Use a different TBAA root for C++ vs C.

This commit changes the root from "Simple C/C++ TBAA" to "Simple C++ TBAA" for
C++.

The problem is that the type name in the TBAA nodes is generated differently
for C vs C++. If we link an IR file for C with an IR file for C++, since they
have the same root and the type names are different, accesses to the two type
nodes will be considered no-alias, even though the two type nodes are from
the same type in a header file.

The fix is to use different roots for C and C++. Types from C will be treated
conservatively in respect to types from C++.

Follow-up commits will change the C root to "Simple C TBAA" plus some mangling
change for C types to make it a little more aggresive.

llvm-svn: 260567