externals: update external GitHub commits refs

Update LVL externals to top of GitHub master branches:

glslang_revision 8f674e821e1e5f628474b21d7fe21af2e86b5fb4
spirv-headers_revision b6dca2397d512e4db62f051a47bb5334d5d44360
spirv-tools_revision c0949703b1264c33df45584efba50a8444b53022

Change-Id: I155c06206267b20c6fbb23fd3692259ad285be1b
diff --git a/external_revisions/glslang_revision b/external_revisions/glslang_revision
index 2822978..64e3226 100644
--- a/external_revisions/glslang_revision
+++ b/external_revisions/glslang_revision
@@ -1 +1 @@
-07939886971c317286dcb831d4ac8feac64a9f43
+8f674e821e1e5f628474b21d7fe21af2e86b5fb4
diff --git a/external_revisions/spirv-headers_revision b/external_revisions/spirv-headers_revision
index b9bc59a..329350d 100644
--- a/external_revisions/spirv-headers_revision
+++ b/external_revisions/spirv-headers_revision
@@ -1 +1 @@
-c470b68225a04965bf87d35e143ae92f831e8110
+b6dca2397d512e4db62f051a47bb5334d5d44360
diff --git a/external_revisions/spirv-tools_revision b/external_revisions/spirv-tools_revision
index bc761a9..7c10d5a 100644
--- a/external_revisions/spirv-tools_revision
+++ b/external_revisions/spirv-tools_revision
@@ -1 +1 @@
-f980d8c6712b6d7d2e406b93a064e2ea40011ca0
+c0949703b1264c33df45584efba50a8444b53022