[clang] Define convergence in C++ languages such as HIP, CUDA, OpenCLusers/ssahasra/clang-convergence-spec

The proposed definition closely follows the existing definition of convergence
in LLVM IR, but using C++ terms to describe language constructs.

There is no undefined behaviour. For each situation, convergence is either fully
specified or implementation-defined.

Two important limitations where LLVM IR requires convergence control tokens to
correctly express the convergence specified here:

1. Some combinations of loops, continue and break statements have different
   convergence specified for the statements inside that region of code, but
   result in the same loops in LLVM IR, thus producing ambiguous convergence in
   LLVM IR.

2. When a divergent condition inside a loop contains a convergent call followed
   by a break statement, these statements are lexically inside the loop, but in
   LLVM IR, they are outside the corresponding CFG loop.

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/docs/conf.py b/clang/docs/conf.py
index 4cee382..8cbf7e0 100644
--- a/clang/docs/conf.py
+++ b/clang/docs/conf.py
@@ -94,6 +94,10 @@ rst_epilog = f"""
 .. |ReleaseNotesTitle| replace:: {in_progress_title} Release Notes
 """
 
+# -- General options for output ------------------------------------------------
+
+numfig = True
+
 # -- Options for HTML output ---------------------------------------------------
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for