Cache function return types

[wmtan]

relval 25 step 3 ran 20% slower per event in ROOT6 that in ROOT5. Much of the problem was traced to FunctionWithDict::finalReturnType(), which constructed a TypeWithDict from the name of the function's return type every time it was called. Simply caching the return types in a concurrent_unordered_map reduced the run time of finalReturnType() by two orders of magnitude, and recovered most or all of the 20% performance deficit.
Please merge this pull request as soon as convenient.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_4_ROOT6_X.

Cache function return types

It involves the following packages:

CommonTools/Utils
FWCore/Utilities

@nclopezo, @monttj, @cmsbuild, @Dr15Jones, @StoyanStoynev, @slava77, @vadler, @ktf can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @wddgit this is something you requested to watch as well.
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