Re-generate {Attribute,ClangCommandLine,Diagnostics}Reference.rst

$ bin/clang-tblgen -gen-diag-docs -I../cfe.src/include \
      -I../cfe.src/include/clang/Basic/ \
      ../cfe.src/include/clang/Basic/Diagnostic.td -o \
      ../cfe.src/docs/DiagnosticsReference.rst && \
bin/clang-tblgen -gen-attr-docs -I../cfe.src/include \
        ../cfe.src/include/clang/Basic/Attr.td -o \
        ../cfe.src/docs/AttributeReference.rst && \
bin/clang-tblgen -gen-opt-docs -I../cfe.src/include \
        -I../cfe.src/include/clang/Driver -I../llvm.src/include \
        ../cfe.src/include/clang/Driver/ClangOptionDocs.td -o \
        ../cfe.src/docs/ClangCommandLineReference.rst

llvm-svn: 370450