msabi: Fix exponential mangling time for certain pathological inputs

Template back references used to be recursively recomputed, add a
memoization cache to cut down on this.

Since there are now two different types of argument maps, rename the
existing TypeBackReferences to FunArgBackReferences, and rename
mangleArgumentType() to mangleFunctionArgumentType().

Fixes PR42091, the input there now takes 50ms instead of 7s to compile.

No intended behavior change.

Differential Revision: https://reviews.llvm.org/D62746

llvm-svn: 362293