[docs] Tell Doxygen to expand LLVM_ALIGNAS to nothing

Summary:
Docs for clang::Decl and clang::TemplateSpecializationType have
not been generated since LLVM_ALIGNAS was added to them.

Tell Doxygen to expand LLVM_ALIGNAS to nothing as described at
https://www.stack.nl/~dimitri/doxygen/manual/preprocessing.html

Reviewers: aaron.ballman, klimek, alexfh

Subscribers: ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D28850

llvm-svn: 292483