Do not check for existence of exported dirs.

Test: header-abi-dumper -o example1.dump tests/input/example1.cpp -I foo -- -x c++
-std=c++11 -I .
-isystem <ToT>/prebuilts/clang/host/linux-x86/clang-3289846/bin/../lib64/clang/3.8/include'

foo does not exist, but header-abi-dumper does not fail. Does not filter
abi instead.

Bug: 37751423

Merged-In: Ia0922f3d3c4ed353fbdbd1c047051888299581ac
Change-Id: Ie6f2d54ec3aa0ccc8c884122dbafc28383870fe5
(cherry picked from commit ea109f70)