cmd/go: document that -msan requires clang

Fixes #14171.

Change-Id: Ie75c1cfd88801618308d472bc04e7fc648c95e0c
Reviewed-on: https://go-review.googlesource.com/19150Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/alldocs.go

@@ -93,7 +93,8 @@ and test commands:
 		Supported only on linux/amd64, freebsd/amd64, darwin/amd64 and windows/amd64.
 	-msan
 		enable interoperation with memory sanitizer.
-		Supported only on linux/amd64.
+		Supported only on linux/amd64,
+		and only with Clang/LLVM as the host C compiler.
 	-v
 		print the names of packages as they are compiled.
 	-work

src/cmd/go/build.go

@@ -72,7 +72,8 @@ and test commands:
 		Supported only on linux/amd64, freebsd/amd64, darwin/amd64 and windows/amd64.
 	-msan
 		enable interoperation with memory sanitizer.
-		Supported only on linux/amd64.
+		Supported only on linux/amd64,
+		and only with Clang/LLVM as the host C compiler.
 	-v
 		print the names of packages as they are compiled.
 	-work