LTO: Correct some doxygen comments about API availability

These look like copy/paste errors, and shouldn't have the "prior to"
qualifier.  Each API was introduced at the given values of
`LTO_API_VERSION`.  The "prior to" in other doxygen comments is because
I couldn't easily differentiate between versions 1 and 2 when I added
these comments.

llvm-svn: 235925
diff --git a/llvm/include/llvm-c/lto.h b/llvm/include/llvm-c/lto.h
index 2f3aac7..9d0d726 100644
--- a/llvm/include/llvm-c/lto.h
+++ b/llvm/include/llvm-c/lto.h
@@ -171,7 +171,7 @@
  * Loads an object file from memory with an extra path argument.
  * Returns NULL on error (check lto_get_error_message() for details).
  *
- * \since prior to LTO_API_VERSION=9
+ * \since LTO_API_VERSION=9
  */
 extern lto_module_t
 lto_module_create_from_memory_with_path(const void* mem, size_t length,
@@ -401,7 +401,7 @@
  *
  * \c cg and \c mod must both be in the same context.
  *
- * \since prior to LTO_API_VERSION=13
+ * \since LTO_API_VERSION=13
  */
 extern void
 lto_codegen_set_module(lto_code_gen_t cg, lto_module_t mod);
@@ -552,7 +552,7 @@
  * Sets if we should run internalize pass during optimization and code
  * generation.
  *
- * \since prior to LTO_API_VERSION=14
+ * \since LTO_API_VERSION=14
  */
 extern void
 lto_codegen_set_should_internalize(lto_code_gen_t cg,