[GCC] GOMP_task: Change argument type of if_cond from int to bool

(libgomp has bool as well)

This was causing a test failure in omp_test_if.c when building with GCC in
Debug mode. I have verified that GCC versions 4.9.2 and 5.3.0 now work and
compile-tested this change with clang 3.7.1 and Intel Compiler 16.0.

Differential Revision: http://reviews.llvm.org/D16921

llvm-svn: 260204