[CMake] In LLVM unified builds, depend on the generation of Clang headers (#70144)

If one is building a LLVM unified build, with Swift as an external
project, SwiftCompilerSources can compile before the `.td` files that
generate Clang headers are finished, which will make the compilation
fail to find some files (the `.inc` files derived from those `.td`).
Make the library that uses the header depend on `clang-tablegen-targets`
to ensure those are done before the library is tried to be used.

This is not a problem in the normal `build-script` builds because in
those cases, the LLVM/Clang headers are build way before Swift is
started to be build.

Author: drodriguez

SwiftCompilerSources/CMakeLists.txt

@@ -263,6 +263,15 @@ else()
   add_library(importedHeaderDependencies "${CMAKE_CURRENT_BINARY_DIR}/HeaderDependencies.cpp")
   target_include_directories(importedHeaderDependencies PRIVATE "${CMAKE_CURRENT_SOURCE_DIR}/../include/swift")
 
+  # When building unified, we need to make sure all the Clang headers are
+  # generated before we try to use them.
+  # When building Swift against an already compiled LLVM/Clang, like
+  # build-script does, this target does not exist, but the headers
+  # are already completely generated at that point.
+  if(TARGET clang-tablegen-targets)
+    add_dependencies(importedHeaderDependencies clang-tablegen-targets)
+  endif()
+
   if(BOOTSTRAPPING_MODE MATCHES "HOSTTOOLS|CROSSCOMPILE")
 
     if (NOT SWIFT_EXEC_FOR_SWIFT_MODULES)