[X86] Add new calling convention that guarantees tail call optimization

When the target option GuaranteedTailCallOpt is specified, calls with
the fastcc calling convention will be transformed into tail calls if
they are in tail position. This diff adds a new calling convention,
tailcc, currently supported only on X86, which behaves the same way as
fastcc, except that the GuaranteedTailCallOpt flag does not need to
enabled in order to enable tail call optimization.

Patch by Dwight Guth <dwight.guth@runtimeverification.com>!

Reviewed By: lebedev.ri, paquette, rnk

Differential Revision: https://reviews.llvm.org/D67855

llvm-svn: 373976