SemaDeclCXX.cpp fails to build with gcc 7 on `could not convert 'MemberLookup' from 'clang::LookupResult' to 'std::optional<clang::LookupResult>'`

[sylvestre]

on linux (ubuntu bionic) with gcc 7.5:

/usr/bin/g++-7 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Itools/clang/lib/Sema -I/build/source/clang/lib/Sema -I/build/source/clang/include -Itools/clang/include -Iinclude -I/build/source/llvm/include -fstack-protector-strong -Wformat -Werror=format-security -Wno-unused-command-line-argument -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-nonnull -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wno-comment -Wno-misleading-indentation -fdiagnostics-color -ffunction-sections -fdata-sections -fdebug-prefix-map=/build/source/build-llvm=build-llvm -fdebug-prefix-map=/build/source/= -no-canonical-prefixes -no-canonical-prefixes -fno-common -Woverloaded-virtual -fno-strict-aliasing -O3 -DNDEBUG  -fno-exceptions -funwind-tables -std=c++1z -MD -MT tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaDeclCXX.cpp.o -MF tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaDeclCXX.cpp.o.d -o tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaDeclCXX.cpp.o -c /build/source/clang/lib/Sema/SemaDeclCXX.cpp
/build/source/clang/lib/Sema/SemaDeclCXX.cpp: In lambda function:
/build/source/clang/lib/Sema/SemaDeclCXX.cpp:17302:12: error: could not convert 'MemberLookup' from 'clang::LookupResult' to 'std::optional<clang::LookupResult>'
     return MemberLookup;
            ^~~~~~~~~~~~
At global scope:
cc1plus: warning: unrecognized command line option '-Wno-unused-command-line-argument'