[clang] Diagnose functions with too many parameters (#104833)

This patch adds a parser check when a function declaration or function
type declaration (in a function pointer declaration, for example) has
too many parameters for `FunctionTypeBits::NumParams` to hold. At the
moment of writing it's a 16-bit-wide bit-field, limiting the number of
parameters at 65536.

The check is added in the parser loop that goes over comma-separated
list of function parameters. This is not the solution Aaron suggested in
https://github.com/llvm/llvm-project/issues/35741#issuecomment-1638086571,
because it was found out that it's quite hard to recover from this
particular error in `GetFullTypeForDeclarator()`. Multiple options were
tried, but all of them led to crashes down the line.

I used LLVM Compile Time Tracker to ensure this does not introduce a
performance regression. I believe changes are in the noise:
https://llvm-compile-time-tracker.com/compare.php?from=de5ea2d122c31e1551654ff506c33df299f351b8&to=424818620766cedb2770e076ee359afeb0cc14ec&stat=instructions:u

Fixes #35741

1 files changed, 11 insertions, 0 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index a8a9d3f..ed5d6ce 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -8025,6 +8025,17 @@ void Parser::ParseParameterDeclarationClause(
         // Consume the keyword.
         ConsumeToken();
       }
+
+      // We can only store so many parameters
+      // Skip until the the end of the parameter list, ignoring
+      // parameters that would overflow.
+      if (ParamInfo.size() == Type::FunctionTypeNumParamsLimit) {
+        Diag(ParmDeclarator.getBeginLoc(),
+             diag::err_function_parameter_limit_exceeded);
+        SkipUntil(tok::r_paren, SkipUntilFlags::StopBeforeMatch);
+        break;
+      }
+
       // Inform the actions module about the parameter declarator, so it gets
       // added to the current scope.
       Decl *Param =