[include-fixer] Add lit-test for relative include path.

[bkramer]

Make sure to watch the buildbots when this goes in, there's a lot of potentially system-dependent shell complexity in the test.

test/include-fixer/Inputs/database_template.json

+[
+{
+  "directory": "test_dir/build",
+  "command": "clang++ -I../include -o bar.o test_dir/src/bar.cpp",
+  "file": "test_dir/src/bar.cpp"
+}
+]

test/include-fixer/Inputs/fake_yaml_db.yaml

 ---
 Name:           foo
 Contexts:
   - ContextType:     Namespace
     ContextName:     a
   - ContextType:     Namespace
     ContextName:     b
 FilePath:        foo.h
 LineNumber:      1
 Type:            Class
 ...
+---
+Name:           bar
+Contexts:
+  - ContextType:     Namespace
+    ContextName:     a
+  - ContextType:     Namespace
+    ContextName:     b
+FilePath:        ../include/bar.h
+LineNumber:      1
+Type:            Class
+...

test/include-fixer/include_path.cpp

+// REQUIRES: shell
+// RUN: mkdir -p %T/include-fixer/include
+// RUN: mkdir -p %T/include-fixer/build
+// RUN: mkdir -p %T/include-fixer/src
+// RUN: sed 's|test_dir|%T/include-fixer|g' %S/Inputs/database_template.json > %T/include-fixer/build/compile_commands.json
+// RUN: cp %S/Inputs/fake_yaml_db.yaml %T/include-fixer/build/fake_yaml_db.yaml
+// RUN: echo 'b::a::bar f;' > %T/include-fixer/src/bar.cpp
+// RUN: touch %T/include-fixer/include/bar.h
+// RUN: cd %T/include-fixer/build
+// RUN: clang-include-fixer -db=yaml -input=fake_yaml_db.yaml -p=. %T/include-fixer/src/bar.cpp
+// RUN: FileCheck -input-file=%T/include-fixer/src/bar.cpp %s
+
+// CHECK: #include "bar.h"
+// CHECK: b::a::bar f;