[Flang][Driver] Enable the -B option

The option provides the search prefix for executables, libraries
and data files.

The option is implemented in the common portion of the Driver and
only needs to be enabled in Flang. Test added is a copy of the
relevant test in Clang.

Author: kiranchandramohan

clang/include/clang/Driver/Options.td

@@ -816,6 +816,7 @@ def _DASH_DASH : Option<["--"], "", KIND_REMAINING_ARGS>,
 def A : JoinedOrSeparate<["-"], "A">, Flags<[RenderJoined]>,
   Group<gfortran_Group>;
 def B : JoinedOrSeparate<["-"], "B">, MetaVarName<"<prefix>">,
+    Visibility<[ClangOption, FlangOption]>,
     HelpText<"Search $prefix$file for executables, libraries, and data files. "
     "If $prefix is a directory, search $prefix/$file">;
 def gcc_install_dir_EQ : Joined<["--"], "gcc-install-dir=">,

flang/test/Driver/B-opt.c

@@ -0,0 +1,12 @@
+// Check -B driver option.
+
+/// Target triple prefix is not detected for -B.
+// RUN: %flang %s -### -o %t.o -target i386-unknown-linux \
+// RUN:     -B %S/Inputs/B_opt_tree/dir1 -fuse-ld=ld 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-B-OPT-TRIPLE %s
+// CHECK-B-OPT-TRIPLE-NOT: "{{.*}}/Inputs/B_opt_tree/dir1{{/|\\\\}}i386-unknown-linux-ld"
+//
+// RUN: %flang %s -### -o %t.o -target i386-unknown-linux \
+// RUN:     -B %S/Inputs/B_opt_tree/dir2 -fuse-ld=ld 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-B-OPT-DIR %s
+// CHECK-B-OPT-DIR: "{{.*}}/Inputs/B_opt_tree/dir2{{/|\\\\}}ld"