Pass -no-canonical-prefixes to clang by default.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 6a495cc2a82b8b2500a5c7c0c5398b7389c8f0a3..f5ee7b3945cb637a6695999f09518683251dece7 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -437,6 +437,14 @@ config("compiler") {
     cflags += [ "-fdebug-prefix-map=$absolute_path=." ]
   }
 
+  # Tells the compiler not to pass the default paths to the tools it invokes.

[Nico, 2017/04/25 23:21:07]

No, it tells the compiler to pass the default paths as relative paths instead of
as absolute paths:

thakis@thakis:~/src/llvm-build-nolibcxx$ diff <(bin/clang -c test.cc -### 2>&1 |
tr " " "\n") <(bin/clang -c test.cc -### -no-canonical-prefixes 2>&1 | tr " "
"\n")
12c12
< /usr/local/google/home/thakis/src/llvm-build-nolibcxx/bin
---
14c14
< "/usr/local/google/home/thakis/src/llvm-build-nolibcxx/bin/clang-4.0"
---
40c40
< "/usr/local/google/home/thakis/src/llvm-build-nolibcxx/lib/clang/5.0.0"
---
52c52
<
"/usr/local/google/home/thakis/src/llvm-build-nolibcxx/lib/clang/5.0.0/include"
---

[Dirk Pranke, 2017/04/25 23:26:45]

Thanks. Updated.

+  # We don't want this because we don't really need it and it can mess up
+  # the goma cache entries. It would be nice if it was on by default in
+  # clang, but it isn't.

[Nico, 2017/04/25 23:29:20]

Can you add a

# TODO(thakis): Figure out if this could be the default behavior.
# TODO(thakis): Expose this in clang-cl if not.

type of comment here?

[Dirk Pranke, 2017/04/26 00:39:30]

Sure.

+  if (is_clang && !is_win) {
+    cflags += [ "-fno-canonical-prefixes" ]

[Nico, 2017/04/25 23:21:07]

It's spelled -no-canonical-prefixes without f I think

[Dirk Pranke, 2017/04/25 23:26:45]

Done.

[shinyak, 2017/04/26 00:52:00]

Yeah, -no-canonical-prefixes is correct.
(Could you upload patch set 2?)

+  }
+
   # C++11 compiler flags setup.
   # ---------------------------
   if (is_linux || is_android || (is_nacl && is_clang)) {