[Driver] Suppress GCC detection under -B

In GCC, if `-B $prefix` is specified, `$prefix` is used to find executable files and startup files.
`$prefix/include` is added as an include search directory.

Clang overloads -B with GCC installation detection semantics which make the
behavior less predictable (due to the "largest GCC version wins" rule) and
interact poorly with --gcc-toolchain (--gcc-toolchain can be overridden by -B).

* `clang++ foo.cpp` detects GCC installation under `/usr`.
* `clang++ --gcc-toolchain=Inputs foo.cpp` detects GCC installation under `Inputs`.
* `clang++ -BA --gcc-toolchain=B foo.cpp` detects GCC installation under A and B and the larger version wins. With this patch, only B is used for detection.
* `clang++ -BA foo.cpp` detects GCC installation under `A` and `/usr`, and the larger GCC version wins. With this patch `A` is not used for detection.

This patch changes -B to drop the GCC detection semantics.  Its executable
searching semantics are preserved.  --gcc-toolchain is the recommended option to
specify the GCC installation detection directory.

(
Note: Clang detects GCC installation in various target dependent directories.
`$sysroot/usr` (sysroot defaults to "") is a common directory used by most targets.
Such a directory is expected to contain something like `lib{,32,64}/gcc{,-cross}/$triple`.
Clang will then construct library/include paths from the directory.
)

Differential Revision: https://reviews.llvm.org/D97993