Make llvm libs export headers.

libLLVM depends on a number of static libraries built from
platform/external/llvm/lib. Make these export headers in order to help
header-abi-dumper to filter out the abi exported by these libraries.

header-abi-dumper's performance improves as it can now avoid recursing
into all the AST nodes which are not exported by these libraries.

This also improves header-abi-linker's performance when it has to combine
the abi dumps from these static libraries into libLLVM's abi dump.

Test: mm -j64.

Change-Id: I70ae2606ab23c778e24d9d465a2e3cde04a0112f
diff --git a/lib/Android.bp b/lib/Android.bp
index 974a447..33b2cf2 100644
--- a/lib/Android.bp
+++ b/lib/Android.bp
@@ -1,6 +1,7 @@
 cc_defaults {
     name: "llvm-lib-defaults",
     defaults: ["llvm-defaults"],
+    export_header_lib_headers: ["llvm-headers"],
     target: {
         windows: {
             enabled: true,