[BuildLibCalls] Name types to identify when 'int' and 'size_t' is assumed. NFC

Lots of BuildLibCalls helpers are using Builder::getInt32Ty to get
a type matching an 'int', and DataLayout::getIntPtrType to get a
type matching 'size_t'. The former is not true for all targets, since
and 'int' isn't always 32 bits. And the latter is a bit weird as well
as the definition of DataLayout::getIntPtrType isn't clearly mapping
it to 'size_t'.

This patch is not aiming at solving any such problems. It is merely
highlighting when a libcall is expecting to use 'int' and 'size_t'
by naming the types as IntTy and SizeTTy when preparing the type
signatures for the emitted libcalls.

Differential Revision: https://reviews.llvm.org/D135064