Revert "[TrailingObjects] Use a different technique to determine if a getDecl"

This reverts commit r256534.

Failed to build on MSVC with error:
clang/ASTMatchers/ASTMatchersInternal.h(572): error C2228: left of '.getDecl' must have class/struct/union
        type is 'add_rvalue_reference<_Ty>::type'

(http://lab.llvm.org:8011/builders/lldb-x86-win7-msvc/builds/13873/steps/build/logs/stdio)

llvm-svn: 256535