[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

llvm/docs/doxygen.cfg.in

@@ -1885,15 +1885,15 @@ ENABLE_PREPROCESSING   = YES
 # The default value is: NO.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-MACRO_EXPANSION        = NO
+MACRO_EXPANSION        = YES
 
 # If the EXPAND_ONLY_PREDEF and MACRO_EXPANSION tags are both set to YES then
 # the macro expansion is limited to the macros specified with the PREDEFINED and
 # EXPAND_AS_DEFINED tags.
 # The default value is: NO.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-EXPAND_ONLY_PREDEF     = NO
+EXPAND_ONLY_PREDEF     = YES
 
 # If the SEARCH_INCLUDES tag is set to YES the includes files in the
 # INCLUDE_PATH will be searched if a #include is found.
@@ -1925,7 +1925,7 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             =
+PREDEFINED             = LLVM_ALIGNAS(x)=
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The