[Coroutines] Use target-agnostic size_t in test

Summary:
Fix a test failure on ARM hosts that was caused by a difference in the type of
size_t, by using a target-agnostic definiton.

Test Plan:
```
clang -cc1 -internal-isystem build/lib/clang/7.0.0/include -nostdsysteminc \
      -std=c++14 -fcoroutines-ts -verify clang/test/SemaCXX/coroutines.cpp \
      -fcxx-exceptions -fexceptions \
      -triple armeb-none-eabi
```

llvm-svn: 325342