Turn on `-Werror` in GCC (#14704)

Cleaned up all the existing GCC warnings via #14507 Turn on `-Werror` to
catch future build problems in CI.

build_tools/cmake/iree_copts.cmake

diff --git a/build_tools/cmake/iree_copts.cmake b/build_tools/cmake/iree_copts.cmake
index d595628..72baaf7 100644
--- a/build_tools/cmake/iree_copts.cmake
+++ b/build_tools/cmake/iree_copts.cmake
@@ -205,9 +205,9 @@
     # intrinsics (#14168).
     "-fno-lax-vector-conversions"
 
-  # TODO(#6959): Enable -Werror once we have a presubmit CI.
   GCC
     "-Wall"
+    "-Werror"
     "-Wno-address-of-packed-member"
     "-Wno-comment"
     "-Wno-format-zero-length"