Performance improvement on LLVM build time (#20)

Summary:
Do a shallow clone using `--shallow-since` flag
in order to spend less time cloning the LLVM repo.
The LLVM repo is huge, which leads to a 400+ MB
directory, and it takes a reasonable time to clone
the repository.

Adding a `--shallow-since` flag using the date
before the desired revision led to a performance
improvement of 4x cloning the repo.

The 8 Oct 2018 date is the day before the desired
revision specified in the code.

## My local tests:

**Without shallow clone:**

```
renato@renato-data-tmp:/tmp$ time git clone https://github.com/llvm-mirror/llvm.git
Cloning into 'llvm'...
remote: Enumerating objects: 166, done.
remote: Counting objects: 100% (166/166), done.
remote: Compressing objects: 100% (134/134), done.
remote: Total 1726648 (delta 52), reused 65 (delta 32), pack-reused 1726482
Receiving objects: 100% (1726648/1726648), 425.35 MiB | 9.27 MiB/s, done.
Resolving deltas: 100% (1418304/1418304), done.
Checking out files: 100% (38646/38646), done.

real	3m40.953s
user	4m38.279s
sys	0m10.589s
```

**With shallow clone:**

```
renato@renato-data-tmp:/tmp$ time git clone --shallow-since 2018-10-08 https://github.com/llvm-mirror/llvm.git llvm-2
Cloning into 'llvm-2'...
remote: Enumerating objects: 168108, done.
remote: Counting objects: 100% (168108/168108), done.
remote: Compressing objects: 100% (61767/61767), done.
Receiving objects: 100% (168108/168108), 91.62 MiB | 9.57 MiB/s, done.
remote: Total 168108 (delta 117253), reused 137482 (delta 104959), pack-reused 0
Resolving deltas: 100% (117253/117253), done.
Checking connectivity: 168108, done.
Checking out files: 100% (38646/38646), done.

real	0m52.412s
user	0m39.649s
sys	0m3.658s
```
Pull Request resolved: #20

Differential Revision: D16242893

Pulled By: dulinriley

fbshipit-source-id: 89c155bea55658c0faab3c5c9886254e2660698f

utils/build_llvm.py

@@ -14,7 +14,12 @@
 import sys
 
 
+# It references the commit day so we can shallow clone
+# and still manage to checkout this specific revision.
+# NOTE: The revision date must be the day before the
+# actual commit date.
 _LLVM_REV = "c179d7b006348005d2da228aed4c3c251590baa3"
+_LLVM_REV_DATE = "2018-10-08" 
 
 
 def parse_args():
@@ -166,7 +171,7 @@ def main():
         print("Cloning LLVM into {}".format(args.llvm_src_dir))
         run_command(
             git
-            + ["clone", "https://github.com/llvm-mirror/llvm.git", args.llvm_src_dir]
+            + ["clone", "--shallow-since", _LLVM_REV_DATE, "https://github.com/llvm-mirror/llvm.git", args.llvm_src_dir]
         )
 
     # Checkout a specific revision in LLVM.