[clang-doc] Silence compiler warning with gcc 7.4 [NFC]

Without the fix gcc 7.4.0 complains with

/data/repo/master/clang-tools-extra/clang-doc/HTMLGenerator.cpp: In member function 'llvm::SmallString<16> clang::doc::{anonymous}::HTMLTag::ToString() const':
/data/repo/master/clang-tools-extra/clang-doc/HTMLGenerator.cpp:165:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
/data/repo/master/clang-tools-extra/clang-doc/HTMLGenerator.cpp: In member function 'bool clang::doc::{anonymous}::HTMLTag::HasInlineChildren() const':
/data/repo/master/clang-tools-extra/clang-doc/HTMLGenerator.cpp:142:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
/data/repo/master/clang-tools-extra/clang-doc/HTMLGenerator.cpp: In member function 'bool clang::doc::{anonymous}::HTMLTag::IsSelfClosing() const':
/data/repo/master/clang-tools-extra/clang-doc/HTMLGenerator.cpp:126:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
cc1plus: all warnings being treated as errors

llvm-svn: 365736