[Clang Tablegen] Add llvm_unreachable() to getModifierName()
[Clang Tablegen] Add llvm_unreachable() to getModifierName()

Fix internal build failure:

../../../ClangDiagnosticsEmitter.cpp -o ClangDiagnosticsEmitter.o
../../../ClangDiagnosticsEmitter.cpp: In function 'llvm::StringRef
{anonymous}::getModifierName({anonymous}::ModifierType)':
../../../ClangDiagnosticsEmitter.cpp:495:1: error: control reaches end of non-void function [-Werror=return-type]
}
^

Build failure triggered by git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@332799 91177308-0d34-0410-b5e6-96231b3b80d8

Differential Revision: https://reviews.llvm.org/D47150

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@332854 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang