clover: Use Clang's diagnostics

clover: Use Clang's diagnostics

Presently errors from frontend are handled only if they occur in
clang::CompilerInvocation::CreateFromArgs(). This patch uses
clang::DiagnosticsEngine to detect errors such as invalid values for
Clang frontend arguments.

Fixes Piglit's cl/program/build/fail/invalid-version-declaration.cl
test.

v2: fix inconsistent code formatting

Signed-off-by: Vedran Miletić <vedran@miletic.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Tested-by: Aaron Watry <awatry@gmail.com>