libclang: Pass Clang install directory to driver via argv[0].

Various driver features, such as the sysroot path detection for Android
targets, rely on being able to find the Clang install directory (look
for callers of `getDriver().getInstalledDir()`). However, the install
directory isn't currently being plumbed through to the driver, which is
conventionally done via the argv[0] passed to the Driver constructor.

It looks like D14695 attempted to fix this by adding another API that
allows specifying the argv[0]. However, rather than requiring every
user of libclang to switch to this API for correct behavior, let's have
the other existing APIs work by default, by using the existing logic in
libclang for finding the install directory.

Differential Revision: https://reviews.llvm.org/D146497