folly | Improve support for clang-cl compiler in folly/Portability.h
folly | Improve support for clang-cl compiler in folly/Portability.h

Summary: `clang-cl` on Windows also defines `__PRETTY_FUNCTION__` which has a format different from `__FUNCSIG__`. Therefore folly breaks users of this macro. Alias should be defined only when "real" MSVC compiler is used.

Reviewed By: akrieger, Orvid, blackm00n

Differential Revision: D17094657

fbshipit-source-id: 4b191a61201704382e6b6e178bcc1c673511c537

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