[InlineCost] Add cl::opt to allow full inline cost to be computed for debugging purposes.

Currently, the inline cost model will bail once the inline cost exceeds the
inline threshold in order to avoid unnecessary compile-time. However, when
debugging it is useful to compute the full cost, so this command line option
is added to override the default behavior.

I took over this work from Chad Rosier (mcrosier@codeaurora.org).

Differential Revision: https://reviews.llvm.org/D35850

llvm-svn: 311371