Add CreateAlignmentAssumption to IRBuilder

Clang CodeGen had a utility function for creating pointer alignment assumptions
using the @llvm.assume intrinsic. This functionality will also be needed by the
inliner (to preserve function-argument alignment attributes when inlining), so
this moves the utility function into IRBuilder where it can be used both by
Clang CodeGen and also other LLVM-level code.
Preserve non-byval pointer alignment attributes using @llvm.assume when inlining

For pointer-typed function arguments, enhanced alignment can be asserted using
the 'align' attribute. When inlining, if this enhanced alignment information is
not otherwise available, preserve it using @llvm.assume-based alignment
assumptions.