[clang][Driver] Add cc1 flag for setting SkipFunctionBodies (#100135)

This is an option set by certain tools (clangd and ASTUnit). Sometimes
there are crashes in clang, unique to this configuration and it's really
hard to provide reproducers without invoking the tool.

Author: kadircet

clang/include/clang/Driver/Options.td

@@ -7534,6 +7534,9 @@ def code_completion_brief_comments : Flag<["-"], "code-completion-brief-comments
 def code_completion_with_fixits : Flag<["-"], "code-completion-with-fixits">,
   HelpText<"Include code completion results which require small fix-its.">,
   MarshallingInfoFlag<FrontendOpts<"CodeCompleteOpts.IncludeFixIts">>;
+def skip_function_bodies : Flag<["-"], "skip-function-bodies">,
+  HelpText<"Skip function bodies when possible">,
+  MarshallingInfoFlag<FrontendOpts<"SkipFunctionBodies">>;
 def disable_free : Flag<["-"], "disable-free">,
   HelpText<"Disable freeing of memory on exit">,
   MarshallingInfoFlag<FrontendOpts<"DisableFree">>;

clang/test/Frontend/skip-function-bodies.cpp

@@ -0,0 +1,13 @@
+// Trivial check to ensure skip-function-bodies flag is propagated.
+//
+// RUN: %clang_cc1 -verify -skip-function-bodies -pedantic-errors %s
+// expected-no-diagnostics
+
+int f() {
+  // normally this should emit some diags, but we're skipping it!
+  this is garbage;
+}
+
+// Make sure we only accept it as a cc1 arg.
+// RUN: not %clang -skip-function-bodies %s 2>&1 | FileCheck %s
+// CHECK: clang: error: unknown argument '-skip-function-bodies'; did you mean '-Xclang -skip-function-bodies'?