Add a switch to not include git commit hash in the built artifacts (#32998)

This PR adds a SWIFT_APPEND_VC_REV Cmake option (in symmetry with LLVM's LLVM_APPEND_VC_REV).

When enabled, lib/Basic/SwiftRevision.inc header contains git commit hash and repository information, e.g.

#define SWIFT_REVISION "ed4cef9b839d4a87618758d8b8705ab66b61917f"
#define SWIFT_REPOSITORY "https://github.com/scentini/swift.git"

This is useful for keeping track of the VCS state that produced a binary. But for local development builds, this information is often ignored and since this file is invalidated by every git commit or git checkout command and it leads to long rebuilds, it's useful to have a switch to disable this behavior. When SWIFT_APPEND_VC_REV is disabled, lib/Basic/SwiftRevision.inc contains:

#undef SWIFT_REVISION
#undef SWIFT_REPOSITORY

Author: hlopko

CMakeLists.txt

@@ -54,6 +54,10 @@ include(CheckSymbolExists)
 # This is primarily to support building smaller or faster project files.
 #
 
+option(SWIFT_APPEND_VC_REV
+  "Embed the version control system revision in Swift"
+  TRUE)
+
 option(SWIFT_INCLUDE_TOOLS
     "Generate build targets for swift tools"
     TRUE)

lib/Basic/CMakeLists.txt

@@ -12,7 +12,12 @@ else()
 endif()
 
 function(generate_revision_inc revision_inc_var name dir)
-  find_first_existing_vc_file("${dir}" ${name}_vc)
+  if(SWIFT_APPEND_VC_REV)
+    # generate_vcs_version_script generates header with only `undef`s
+    # inside when source directory doesn't exist.
+    find_first_existing_vc_file("${dir}" ${name}_vc)
+    set(dir_when_append_enabled ${dir})
+  endif()
 
   # Create custom target to generate the VC revision include.
   set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/${name}Revision.inc")
@@ -22,7 +27,7 @@ function(generate_revision_inc revision_inc_var name dir)
   add_custom_command(OUTPUT "${version_inc}"
     DEPENDS "${${name}_vc}" "${generate_vcs_version_script}"
     COMMAND ${CMAKE_COMMAND} "-DNAMES=$<UPPER_CASE:${name}>"
-                             "-D$<UPPER_CASE:${name}>_SOURCE_DIR=${dir}"
+                             "-D$<UPPER_CASE:${name}>_SOURCE_DIR=${dir_when_append_enabled}"
                              "-DHEADER_FILE=${version_inc}"
                              -P "${generate_vcs_version_script}")