Use the correct project name in README.txt

[gbedwell]

Summary

I happened to notice this when browsing one of the github mirrors.

Per llvm.org: "The name "LLVM" itself is not an acronym; it is the full name of the project."

Normally for a simple documentation cleanup like this I'd forgo the review process and commit directly, but as it is the first line of the first file many new contributors will see, I figured better to be safe ;).

[lattner]

How about changing it to "LLVM Compiler Infrastructure". Accepted with that tweak, thanks!

[gbedwell]

In D47796#1123026, @lattner wrote:

How about changing it to "LLVM Compiler Infrastructure". Accepted with that tweak, thanks!

Thanks Chris! I went with the definite article to match the heading on llvm.org (plus that happens to make the commit touch exactly half the number of lines of my original version, so it's far less invasive ;)).

README.txt

-Low Level Virtual Machine (LLVM)
-================================
+LLVM
+====
 
 This directory and its subdirectories contain source code for LLVM,
 a toolkit for the construction of highly optimized compilers,
 optimizers, and runtime environments.
 
 LLVM is open source software. You may freely distribute it under the terms of
 the license agreement found in LICENSE.txt.
 
 Please see the documentation provided in docs/ for further
 assistance with LLVM, and in particular docs/GettingStarted.rst for getting