Use C++14 `auto` return type inference in pattern_matcher.h.
Use C++14 `auto` return type inference in pattern_matcher.h.

In C++11, we had to say

  auto fn() -> decltype(...) { ... }

but in C++14, we can simply say

  auto fn() { ... }

Do this in pattern_matcher.h.

PiperOrigin-RevId: 327085487
Change-Id: Ida65f273a45c8ac7a59a1876f1db1e5fc895a8f6

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