[ADT] Simplify llvm::bit_cast (NFC)

[ADT] Simplify llvm::bit_cast (NFC)

This patch removes macro tricks to check GCC versions.

The commit message from 19262fc5966ab569f21f3d440f8b001bca666f17
states that "is_trivially_copyable is only in GCC 5.1 and later".
Note that we now require GCC 7.1 or higher.

Since both std::is_trivially_constructible and
std::is_trivially_copyable are C++11 features, and we now require
C++17, we probably don't need to worry about the availability of the
C++11 features.

Differential Revision: https://reviews.llvm.org/D132330