[orc-rt] Add a coding conventions doc (#208623)

Add docs/CodingConventions.md, recording that ORC-RT follows the LLVM
Coding Standards and describing the C API naming conventions.

orc-rt/docs/CodingConventions.md

@@ -0,0 +1,61 @@
+# ORC-RT Coding Conventions
+
+## Overview
+
+ORC-RT is part of the LLVM project and generally follows the
+[LLVM Coding Standards](https://llvm.org/docs/CodingStandards.html), including
+LLVM's naming conventions for C++ code: `PascalCase` for types, values, and
+variables, and `camelBack` (lower-camelCase) for functions.
+
+This document records conventions that are specific to ORC-RT, or that need
+restating because they are not obvious from the LLVM standards alone. Where this
+document is silent, follow the LLVM Coding Standards.
+
+## C API naming
+
+The C API headers (under `include/orc-rt-c/`) cannot use C++ namespaces, so
+namespacing is expressed through symbol-name prefixes, while the *casing* of
+each name still follows the LLVM naming conventions used elsewhere in the
+project.
+
+The rules are:
+
+- **`orc_rt_` is the top-level C namespace.** Every public C symbol begins with
+  it.
+- **Nested namespaces are additional lower-case, underscore-separated
+  segments.** For example, `orc_rt_log_` is the nested namespace for the logging
+  API.
+- **Types, values, and variables are `PascalCase`** (as in LLVM), appended to
+  their namespace prefix.
+- **Functions are `camelBack`** (as in LLVM), appended to their namespace
+  prefix.
+- **Enumerators are `PascalCase` values, scoped by prefixing them with their
+  enclosing type's name** (C has no scoped enums), joined with an underscore.
+- **Macros are `UPPER_CASE`**, as in LLVM and C generally.
+
+A PascalCase type name may itself act as a scope for its "member" functions,
+exactly like a nested namespace does; the two are indistinguishable in the
+flattened C name, and both are spelled with a trailing underscore.
+
+### Examples
+
+| Symbol                        | Kind     |
+|-------------------------------|----------|
+| `orc_rt_ErrorRef`             | type     |
+| `orc_rt_Error_getTypeId`      | function |
+| `orc_rt_StringError_create`   | function |
+| `orc_rt_log_Category`         | type     |
+| `orc_rt_log_Category_General` | value    |
+| `orc_rt_log_formatCheck`      | function |
+| `ORC_RT_LOG`                  | macro    |
+
+Reading these: `orc_rt_Error_getTypeId` is the `getTypeId` function scoped to
+the `Error` type; `orc_rt_log_formatCheck` is the `formatCheck` function in the
+`log` namespace; and `orc_rt_log_Category_General` is the `General` value of the
+`Category` type in the `log` namespace.
+
+## C++ code
+
+C++ code follows the LLVM Coding Standards directly: symbols live in real
+namespaces (e.g. `orc_rt`), types/values/variables are `PascalCase`, functions
+are `camelBack`, and macros are `UPPER_CASE`.

orc-rt/docs/index.md

@@ -9,6 +9,7 @@ The ORC runtime provides executor-side support code for the LLVM ORC APIs.
    :titlesonly:
 
    Building-orc-rt
+   CodingConventions
 ```
 
 ### Current Status