[pseudo] Add xfail tests for a simple-declaration/function-definition ambiguity

[sammccall]

Summary

I expect to eliminate this ambiguity at the grammar level by use of guards,
because it interferes with brace-based error recvoery.

[hokein]

clang-tools-extra/pseudo/test/cxx/declarator-function.cpp

13

nit: the line seems to be broken by the clang-format.

clang-tools-extra/pseudo/test/cxx/declarator-var.cpp

11

why we need two lines for function-definition?
because of the undeterministic order of the ambiguous results (we don't know whether function-def comes first or the var-decl comes first)?

[sammccall]

clang-tools-extra/pseudo/test/cxx/declarator-var.cpp

11

Right, we want to forbid function-definition whether it's first or second.

In a more complicated example I might use -implicit-check-not but I think this is clearer: hiding assertions in the FileCheck invocation is non-obvious.

clang-tools-extra/pseudo/test/cxx/declarator-function.cpp

+// The standard grammar allows an init-list with any declarator, including
+// a function declarator. This creates an ambiguity where a function-definition
+// is misparsed as a simple-declaration.
+// FIXME: eliminate this false parse.
+// XFAIL: *
+
+// RUN: clang-pseudo -grammar=%cxx-bnf-file -source=%s --print-forest | FileCheck %s
+void s(){};
+// CHECK-NOT:      simple-declaration
+// CHECK:          function-definition := decl-specifier-seq declarator
+// function-body CHECK-NOT:      simple-declaration

clang-tools-extra/pseudo/test/cxx/declarator-var.cpp

+// The standard grammar allows an function-body to use any declarator, including
+// a non-function declarator. This creates an ambiguity where a
+// simple-declaration is misparsed as a function-definition.
+// FIXME: eliminate this false parse.
+// XFAIL: *
+
+// RUN: clang-pseudo -grammar=%cxx-bnf-file -source=%s --print-forest | FileCheck %s
+void (*s)(){};
+// CHECK-NOT:      function-definition
+// CHECK:          init-declarator := declarator initializer
+// CHECK-NOT:      function-definition