Add doc and release notes for cc.preprocess()

docs/markdown/snippets/preprocess.md

@@ -0,0 +1,14 @@
+## New method to preprocess source files
+
+Compiler object has a new `preprocess()` method. It is supported by all C/C++
+compilers. It preprocess sources without compiling them.
+
+The preprocessor will receive the same arguments (include directories, defines,
+etc) as with normal compilation. That includes for example args added with
+`add_project_arguments()`, or on the command line with `-Dc_args=-DFOO`.
+
+```meson
+cc = meson.get_compiler('c')
+pp_files = cc.preprocess('foo.c', 'bar.c', output: '@PLAINNAME@')
+exe = executable('app', pp_files)
+```

docs/yaml/objects/compiler.yaml

@@ -586,3 +586,25 @@ methods:
     gcc or msvc, but use the same argument syntax as one of those two compilers
     such as clang or icc, especially when they use different syntax on different
     operating systems.
+
+- name: preprocess
+  returns: list[custom_idx]
+  since: 0.64.0
+  description: |
+    Preprocess a list of source files but do not compile them. The preprocessor
+    will receive the same arguments (include directories, defines, etc) as with
+    normal compilation. That includes for example args added with
+    `add_project_arguments()`, or on the command line with `-Dc_args=-DFOO`.
+  varargs_inherit: _build_target_base
+  kwargs:
+    output:
+      type: str
+      description: |
+        Template for name of preprocessed files: `@PLAINNAME@` is replaced by
+        the source filename and `@BASENAME@` is replaced by the source filename
+        without its extension.
+    compile_args:
+      type: list[str]
+      description: |
+        Extra flags to pass to the preprocessor
+