Fix build with Qt 5.9.1 and clang

Summary:
Qt 5.9.1 expands Q_REQUIRED_RESULT to the [[nodiscard]] attribute
(see commit d254d8c19 in qtbase). This breaks the build with clang
compilers because [[nodiscard]] cannot be used at the end of a function
declaration:

error: 'nodiscard' attribute cannot be applied to types

For the record, GCC just issues a 'attribute ignored' warning when
[[nodiscard]] is used at the end of a declaration

BUG: 382135
FIXED-IN: 5.1.2