[cmake] Remove standalone clang config code for building clang separate from llvm/swift.

We do not support this now. When we do, code can be added back.

rdar://26154980

Author: gottesmm

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -120,22 +120,14 @@ macro(swift_common_standalone_build_config_clang product is_cross_compiling)
 
   if(NOT EXISTS "${${product}_PATH_TO_CLANG_SOURCE}/include/clang/AST/Decl.h")
     message(FATAL_ERROR "Please set ${product}_PATH_TO_CLANG_SOURCE to the root directory of Clang's source code.")
-  else()
-    get_filename_component(CLANG_MAIN_SRC_DIR ${${product}_PATH_TO_CLANG_SOURCE}
-      ABSOLUTE)
   endif()
+  get_filename_component(CLANG_MAIN_SRC_DIR ${${product}_PATH_TO_CLANG_SOURCE}
+    ABSOLUTE)
 
-  if(EXISTS "${${product}_PATH_TO_CLANG_BUILD}/include/clang/Basic/Version.inc")
-    set(CLANG_BUILD_INCLUDE_DIR "${${product}_PATH_TO_CLANG_BUILD}/include")
-  elseif(EXISTS "${${product}_PATH_TO_CLANG_BUILD}/tools/clang/include/clang/Basic/Version.inc")
-    set(CLANG_BUILD_INCLUDE_DIR "${${product}_PATH_TO_CLANG_BUILD}/tools/clang/include")
-  else()
+  if(NOT EXISTS "${${product}_PATH_TO_CLANG_BUILD}/tools/clang/include/clang/Basic/Version.inc")
     message(FATAL_ERROR "Please set ${product}_PATH_TO_CLANG_BUILD to a directory containing a Clang build.")
   endif()
-  if(CLANG_MAIN_INCLUDE_DIR)
-    get_filename_component(CLANG_MAIN_SRC_DIR ${${product}_PATH_TO_CLANG_SOURCE}
-      ABSOLUTE)
-  endif()
+  set(CLANG_BUILD_INCLUDE_DIR "${${product}_PATH_TO_CLANG_BUILD}/tools/clang/include")
 
   if (NOT ${is_cross_compiling})
     set(${product}_NATIVE_CLANG_TOOLS_PATH "${PATH_TO_LLVM_TOOLS_BINARY_DIR}")