Explicitly specify -fintegrated-as to clang/test/Driver/compilation_database.c test case.

It appears that this test assumes that the toolchain utilizes the integrated
assembler by default, since the expected output in the CHECKs are
compilation_database.o.

However, this test fails on AIX as AIX does not utilize the integrated assembler.
On AIX, the output instead is of the form /tmp/compilation_database-*.s.
Thus, this patch explicitly adds the -fintegrated-as option to match the
assumption that the integrated assembler is used by default.

Differential Revision: https://reviews.llvm.org/D110431