Add github.com/nlohmann/json as a third_party dependency

Required by cppdap, which is used by the new Vulkan debugger.

Bug: b/145351270
Change-Id: Ibf65a63ed3c508722993578c57d72b1884a06ff9
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38888
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

.gitmodules

diff --git a/.gitmodules b/.gitmodules
index 4fe1ebb..a94ba42 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -2,6 +2,10 @@
 	path = third_party/googletest
 	url = https://github.com/google/googletest.git
 
+[submodule "third_party/json"]
+	path = third_party/json
+	url = https://github.com/nlohmann/json.git
+
 [submodule "third_party/libbacktrace/src"]
 	path = third_party/libbacktrace/src
 	url = https://github.com/ianlancetaylor/libbacktrace.git

third_party/json

diff --git a/third_party/json b/third_party/json
new file mode 160000
index 0000000..ed55414
--- /dev/null
+++ b/third_party/json
@@ -0,0 +1 @@
+Subproject commit ed5541440a36bf7dc1a544f9a84fa3e5ae97b71f