Revert "We allow implicit function declarations as an extension in all C dialects. Remove OpenCL special case."

Discussed on cfe-commits (Week-of-Mon-20180820), this change leads to
the generation of invalid IR for OpenCL without giving an error.
Therefore, the conclusion was to revert.

llvm-svn: 342885
diff --git a/clang/test/SemaOpenCL/to_addr_builtin.cl b/clang/test/SemaOpenCL/to_addr_builtin.cl
index 9b95ff4..fb21a15 100644
--- a/clang/test/SemaOpenCL/to_addr_builtin.cl
+++ b/clang/test/SemaOpenCL/to_addr_builtin.cl
@@ -10,7 +10,7 @@
 
   glob = to_global(glob, loc);
 #if __OPENCL_C_VERSION__ < CL_VERSION_2_0
-  // expected-warning@-2{{implicit declaration of function 'to_global' is invalid in OpenCL}}
+  // expected-error@-2{{implicit declaration of function 'to_global' is invalid in OpenCL}}
   // expected-warning@-3{{incompatible integer to pointer conversion assigning to '__global int *' from 'int'}}
 #else
   // expected-error@-5{{invalid number of arguments to function: 'to_global'}}