[ADT] Normalize empty triple components

LLVM triple normalization is handling "unknown" and empty components
differently; for example given "x86_64-unknown-linux-gnu" and
"x86_64-linux-gnu" which should be equivalent, triple normalization
returns "x86_64-unknown-linux-gnu" and "x86_64--linux-gnu". autoconf's
config.sub returns "x86_64-unknown-linux-gnu" for both
"x86_64-linux-gnu" and "x86_64-unknown-linux-gnu". This changes the
triple normalization to behave the same way, replacing empty triple
components with "unknown".

This addresses PR37129.

Differential Revision: https://reviews.llvm.org/D50219

llvm-svn: 339294
diff --git a/clang/test/Driver/baremetal.cpp b/clang/test/Driver/baremetal.cpp
index 2e93734..68031fe 100644
--- a/clang/test/Driver/baremetal.cpp
+++ b/clang/test/Driver/baremetal.cpp
@@ -4,7 +4,7 @@
 // RUN:     -L some/directory/user/asked/for \
 // RUN:     --sysroot=%S/Inputs/baremetal_arm \
 // RUN:   | FileCheck --check-prefix=CHECK-V6M-C %s
-// CHECK-V6M-C: "[[PREFIX_DIR:.*]]{{[/\\]+}}{{[^/^\\]+}}{{[/\\]+}}clang{{.*}}" "-cc1" "-triple" "thumbv6m-none--eabi"
+// CHECK-V6M-C: "[[PREFIX_DIR:.*]]{{[/\\]+}}{{[^/^\\]+}}{{[/\\]+}}clang{{.*}}" "-cc1" "-triple" "thumbv6m-none-unknown-eabi"
 // CHECK-V6M-C-SAME: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK-V6M-C-SAME: "-isysroot" "[[SYSROOT:[^"]*]]"
 // CHECK-V6M-C-SAME: "-internal-isystem" "[[SYSROOT]]{{[/\\]+}}include{{[/\\]+}}c++{{[/\\]+}}v1"