[clang] Add getSignedSizeType method

C11 standard refers to the signed counterpart of the type size_t in
the paragraph 7.21.6.1 where it defines d, i, o, u, x, or x conversion specifiers
(in printf format string).
In Clang there is a FIXME (in lib/Analysis/PrintfFormatString.cpp) for this case
(which is not handled correctly at the moment).
This diff adds getSignedSizeType method to TargetInfo and exposes it 
in ASTContext similarly to how it is done for getSizeType.
lib/Analysis/PrintfFormatString.cpp will be changed in a separate commit.

Differential revision: https://reviews.llvm.org/D35378

Test plan: make check-all

llvm-svn: 308037