[docs] Tell Doxygen to expand LLVM_ALIGNAS to nothing

[malcolm.parsons]

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

[aaron.ballman]

LGTM, though I freely admit that I am no doxygen expert (from reading the docs, this looks like the sensible approach though).

[malcolm.parsons]

In D28850#649462, @aaron.ballman wrote:

LGTM, though I freely admit that I am no doxygen expert (from reading the docs, this looks like the sensible approach though).

I couldn't find a code owner for docs to ask for review.

There are other doxygen.cfg.in files in llvm:

docs/doxygen.cfg.in
tools/clang/tools/extra/docs/doxygen.cfg.in
tools/lldb/docs/doxygen.cfg.in

Should I change them all?

docs/doxygen.cfg.in

 
 # If the MACRO_EXPANSION tag is set to YES doxygen will expand all macro names
 # in the source code. If set to NO only conditional compilation will be
 # performed. Macro expansion can be done in a controlled way by setting
 # EXPAND_ONLY_PREDEF to 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.
 # The default value is: YES.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
 SEARCH_INCLUDES        = YES
 

 # The PREDEFINED tag can be used to specify one or more macro names that are
 # defined before the preprocessor is started (similar to the -D option of e.g.
 # gcc). The argument of the tag is a list of macros of the form: name or
 # name=definition (no spaces). If the definition and the "=" are omitted, "=1"
 # is assumed. To prevent a macro definition from being undefined via #undef or
 # 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
 # macro definition that is found in the sources will be used. Use the PREDEFINED
 # tag if you want to use a different macro definition that overrules the
 # definition found in the source code.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.