cc_toolchain should use compiler_files for CppCompileActions

[hlopko]

Currently, cc_toolchain uses all_files attribute for compilation actions, which is unnecessarily big. This effectively makes compiler_files unused.

We should use compiler_files instead.

Note: update cc_toolchain docs after fixing this, docs reference this issue.

[benjaminp]

This issue is about making the following change, right?

diff --git a/src/main/java/com/google/devtools/build/lib/bazel/rules/cpp/BazelCppSemantics.java b/src/main/java/com/google/devtools/build/lib/bazel/rules/cpp/BazelCppSemantics.java
index f266feea0e..917c4a43ce 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/rules/cpp/BazelCppSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/rules/cpp/BazelCppSemantics.java
@@ -47,9 +47,7 @@ public class BazelCppSemantics implements AspectLegalCppSemantics {
       FeatureConfiguration featureConfiguration,
       CppCompileActionBuilder actionBuilder) {
     actionBuilder
-        // Because Bazel does not support include scanning, we need the entire crosstool filegroup,
-        // including header files, as opposed to just the "compile" filegroup.
-        .addTransitiveMandatoryInputs(actionBuilder.getToolchain().getAllFiles())
+        .addTransitiveMandatoryInputs(actionBuilder.getToolchain().getCompilerFiles())
         .setShouldScanIncludes(false);
   }

Should we add an --incompatible_ flag for this? I'd like to see it happen.